John Newton (August 25, 1822 – May 1, 1895) was a career engineer officer in the United States Army, a Union general in the American Civil War, and Chief of the Corps of Engineers. Newton was born in Norfolk, Virginia. He ranked second in the United States Military Academy class of 1842 and was commissioned in the Corps of Engineers. He taught engineering at the Military Academy (1843–46) and constructed fortifications along the Atlantic coast and Great Lakes (1846–52). He was a member of a special Gulf Coast defense board (1856) and Chief Engineer, Utah Expedition (1858). At Gettysburg, he replaced the slain Maj. Gen. John F. Reynolds in command of the I Corps and led it through the defense of Pickett's Charge. He retained command of I Corps until the Army of the Potomac was reorganized in 1864 for Lt. Gen. Ulysses S. Grant's Overland Campaign. The I Corps was dissolved, and Newton was sent to the Army of the Cumberland.
The Newton's U.S. 1st Corps Headquarters Marker is located on Pleasonton Avenue, on the south or right side of the road is traveling due east, south of Gettysburg. The upright cannon is roadside and northeast of the Pennsylvania State Monument, in front of and to the right of the Roller and Storage Building. 
Parking is available all up and down Pleasonton Avenue at enlarged shoulder spaces along the road. Some good advice would be to NOT park on anything green like grass or weeds as you will be ticketed by park police. I visited this monument on Thursday, July 5, 2012 in the afternoon. I was at an elevation of about 575 feet, ASL. I used a Canon PowerShot 14.1 Megapixel, SX210 IS digital camera for the photos.
The monument was completed in June of 1913 according to the NPS/NRHP narrative and a few of my other sources. I know for sure the work was done under the direction of the Gettysburg National Park Commission (established by the United States Department of War), after they took over the administration of the park from the Gettysburg Battlefield Memorial Association (whose funds had expired) on March 3, 1893, and whose stewardship was then transferred to the National Park Service in 1933, (which entity still operates the park today). SOURCE
Description
Draw the Sword, with descriptive help from the NPS site (and me), offers the following description: Consists of cast iron cannon tube, 4.5-inch rifled siege gun, mounted on a granite base. Coursed, rough-hewn base, 3’2?sq, 2’7?H. Wrought Iron cannon tube mounted upright on base. Overall 10’6?H. Bronze inscription tablet on east side facing park avenue and public road.
This monument and the other 9 for the Union were designed by designed by architect Colonel Emmor Bradley Cope (July 23, 1834 - May 28, 1927, working for the War Department). He designed pretty much every tablet for both the Union & Rebel armies, each one distinct, w/ several different varieties as well as these headquarters markers. He is buried in Evergreen Cemetery 
in Gettysburg, as well he should be as this park and most of the things seen here today were designed by him and as such, are his legacy.
The gun/cannon is a 4.5-inch iron Ordnance Rifles (also called a Siege Rifle), Model of 1861. My usual SITE for all things weaponry at Gettysburg usually has the registry information which can be found on the muzzle stamp. This thing is so high, I had no chance of reading it but this site has that information. The information in red is not part of the muzzle stamp but additional information cultivated by my source. That information reads as follows:
F.P.F. ......No. 112 ......1866 ......C.C. ......3547 ......FDY #2428 ......GRVS 9RH
F.P.F represents Fort Pitt Foundry, Pittsburgh PA, the place where this cannon was manufactured. No. 112 Refers to the Army registration number, a way for the military to keep track of the guns when they take receipt of them from the various Union foundries. C.C. are the initials of the Union Ordnance Officer responsible for inspecting and accepting the final product to make sure it met all standards before the Army took receipt of the weapon. I believe those initials stand for Clifton Comly who worked as an ordnance officer from 1863 to 1889. FDY #2428 refers to the foundry's internal control number of the cannon for their inventory purposes only. 1866 refers to the production date, 1866. This weapon could not have seen action here in 1863. 3547 refers to the weight of the firing tube. Each gun is usually very unique and has its own weight which distinguishes it from every other gun, like a fingerprint. From my own experience after examining many of the cannons @ Gettysburg, this is the heaviest of the cannons I have found so far, more than 3 times as heavy as the common 12-pounder Napoleons found here. GRVS 9RH indicates this cannon was rifled or had grooves put into the bore. This refers to the number of rifling grooves, and if they were a right or left hand twist. Accordingly, this was 9 right hand twists. Rifling made the weapon much more accurate.
About the Foundry
The Fort Pitt Foundry was a nineteenth century iron foundry in Pittsburgh, Pennsylvania. It was originally established at Fifth Avenue and Smithfield Street in 1804 by Joseph McClurg, grandfather of Joseph W. McClurg. It was later moved to the area of Pittsburgh now known as the Strip District at 12th and Etna. It was an early producer of ordinance for the United States, and manufactured cannonballs for Commodore Oliver Hazard Perry's forces in the War of 1812. It was best known for the manufacture of large cannon. One of the largest was a 20 inch bore Rodman Gun, a large black powder, smoothbore, muzzle-loading coastal defense gun. The foundry was closed after the Civil War ended. It was eventually sold to a rival in 1878.
SOURCE
About the Gun (4.5-inch Siege Rifle)
The 4.5-inch siege rifle looks like a larger version of the 3-inch ordnance rifle and it is often called a 4.5-ordnance rifle. However, the 4.5-inch Siege rifle was of conventional cast iron construction and did not use the welded wrought iron construction of the 3-inch ordnance rifle. The 4.5-inch siege rifle fired shells weighing about 30 pounds (depending on the specific type of shell). It weighed 3,450 pounds and was 133 inches long. The gun’s only vice was that it suffered from excessive erosion of the vent caused by the hot gasses flowing through the vent when the gun was fired. The vent could be too large to fire the gun after 400 discharges. This problem could be remedied by insertion of a copper vent piece (bouche). In addition to its use as siege artillery, two batteries of 4.5- siege rifles (8 guns total) accompanied the Army of the Potomac as “heavy” field artillery between 1862 and 1864. The big guns were intended for long range firing against Confederate artillery. Although the guns showed very good mobility, they saw little action
SOURCE
About the Inspector
Through a New York Times obituary I learned Major Clifton Comly was president of the Ordnance Department of the Department of the East. It seems, according to a crude medical explanation, the Major suffered a stroke, paralysis and eventual cardiac complications and died at night, before midnight on an unknown date in 1889. SOURCE He was on duty at the time at Sandy Hook Proving Station on a Tuesday when his attack occurred. From the Officer Register of the Officers and Cadets of the U.S. Military Academy at West Point, New York (SOURCE), I learned Comly came from Ohio and was admitted to the military academy on July 1, 1858, exactly five years before Gettysburg. He was 17 years old and one month at the time of his admission. His grades or merit are listed and ironically, ordnance was his second to worst subject. He was in the first graduating class of 1862 and there were 28 members in his class. Other more ambiguous and esoteric sources confirmed his D.O.B. as May 31, 1841 in Dayton, Ohio. He died April 17, 1894 (which was indeed a Tuesday which jibes with the N.Y Times report) He died at the New York Arsenal, Governor's Island, New York, N.Y. This information came from an excellent page which listed his entire family tree and genealogy. SOURCE
The inscription on the curved bronze plate reads:
Army of the Potomac
1st. Corps Headquarters
Major General
John Newton
July 2,3,4 1863
The Newton's U.S. 1st Corps Headquarters Marker is a contributing feature to the Gettysburg National Military Park Historic District which is nationally significant under NR Criteria A, B, C & D. Areas of Significance: Military, Politics/Government, Landscape Architecture, Conservation, Archeology-Historic. Period of Significance: 1863-1938. The original National Register Nomination was approved by the Keeper March 19, 1975. An update to this nomination was approved by the Keeper on January 23, 2004. The monument is identified as structure number MN452.
From the Nomination Form:
1 of 10 Civil War US Headquarters Markers in Park of the GBMA Era (1895-1927). Marks the location of Newton's 1st Corps US Headquarters during Battle of Gettysburg, July 1-3, 1863. Located south side Pleasonton Ave, near Park's Roller and Storage Building.
Short Physical Description:
Coursed, rough-hewn base, 3'2"sq, 2'7"H. Wrought Iron cannon tube mounted upright on base. Overall 10'6"H. Bronze inscription tablet on east side facing park avenue and public road.
Long Physical Description:
N/A
My Sources
1.
NRHP Nomination Form
2.
Stone Sentinels
3.
Virtual Gettysburg
4.
Draw the Sword
5.
Historical Marker Database
6.
Wikipedia