The 2nd United States Corps was also known as Hancock's Corps. The unit was commanded by Winfield Scott Hancock (February 14, 1824 – February 9, 1886) was a career U.S. Army officer and the Democratic nominee for President of the United States in 1880. He served with distinction in the Army for four decades, including service in the Mexican-American War and as a Union general in the American Civil War. Known to his Army colleagues as "Hancock the Superb", he was noted in particular for his personal leadership at the Battle of Gettysburg in 1863. One military historian wrote, "No other Union general at Gettysburg dominated men by the sheer force of their presence more completely than Hancock."
The Hancock's U.S. 2nd Corps Headquarters Marker is located on Pleasonton Avenue, on the north or left side of the road is traveling due east, south of Gettysburg. The upright cannon is roadside and north of the Pennsylvania State Monument, no doubt basking in its mid-day shadow. 
Parking is available all up and down Pleasonton Avenue, Hancock Avenue, and Humphreys Avenues 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 approximately 6:15 P.M. 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: One of ten Civil War US Headquarter Markers in the Park. Coursed, rough-hewn base, 3’2" squared x 2’7' in height. Wrought Iron cannon tube mounted upright on base with excised Trefoil Corps symbol at cannon base. Overall 10’6?H. Bronze inscription tablet on west side, facing public roads and park avenues.
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. 90 ......1863 ......R.H.K.W. ......3557 ......FDY #1788 ......GRVS 9RH
F.P.F represents Fort Pitt Foundry, Pittsburgh PA, the place where this cannon was manufactured. No. 90 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. R.H.K.W refers Robert Henry Kirkwood Whiteley, the military officer responsible for inspecting and accepting the final product to make sure it met all standards before the Army took receipt of the weapon. FDY #1788 refers to the foundry's internal control number of the cannon for their inventory purposes only. 1863 refers to the production date, 1863. Theoretically, this weapon could have seen action here in 1863. 3557 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
Whiteley was appointed to the U.S. Military Academy and graduated 13th in his class in 1830.
While a student at West Point, Cadet Whiteley began a friendship with Robert E. Lee who was one year his senior. This relationship culminated with the offer by Lee, at the beginning of the Civil War, to be commandant of the Confederate Army Ordnance Corps with the rank of Major General. Then Lt Col. Whiteley declined the offer in order to remain with the Union Army. Lt. Whiteley married Hester Dodson on November 30, 1830. While moving from post to post the Whiteleys had ten children. Brigadier General Whiteley retired from the U.S. Army after 45 years of service (1830 - 1875) and died in Baltimore, MD on June 9, 1896. SOURCE
The inscription on the curved bronze plate reads:
Army of the Potomac
2d. Corps Headquarters
Major General
Winfield S. Hancock
July 1,2,3,4 1863
The Hancock's U.S. 2nd 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 MN453.
From the Nomination Form:
1 of 10 Civil War US Headquarter Markers in Park of the War Dept. Era (1895-1927). Marks the location of Howard's 11th Corps US Headquarters during Battle of Gettysburg, July 1-4, 1863. Located east side of Baltimore Pike on East Cemetery Hill.
Short Physical Description:
Coursed, rough-hewn base, 3'2"sq, 2'7"H. Wrought Iron cannon tube mounted upright on base w/ excised Trefoil Corps symbol at cannon base. Overall 10'6"H. Bronze inscription tablet on west side, facing public roads and park avenues.
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