There are hundreds of cannons at Gettysburg, many of them flanking monuments and markers. Some monuments, like this one, utilize only the firing tube and then turn that into the monument. The canon tube is located on West Confederate Avenue, just past the intersection of Section 4 West Confederate Avenue and Millerstown Road, on the right when traveling south on West Confederate Avenue, near Pitzer's Woods, and located near stop six of the driving tour of Gettysburg National Military Park, across the street from Longstreet Observation tower. The monument faces the east. 
Parking can be had at the observation tower. The last couple or so slots are for buses so don't park there. Please do not park on the grass, park on the side of the road if you are not using the lot. I cannot emphasize that enough! You will be ticketed. I visited this monument on Saturday, March 10, 2012 at 3:43 PM, just before the clocks were set ahead for the Spring. I was at a 585 foot ASL elevation.
The firing tube is no longer shiny but green as the bronze has long since oxidized. My usual SOURCE for cannon documentation provided me with the following line of information about this specific cannon, written on the outer muzzle, which I did not have a ladder to climb and then read:
Columbus - Reg# 21 - INSP FCH - FDY# 35 - YR 63 - WT 1210
Columbus refers to the Columbus Arsenal, a foundry out of Columbus GA and the place where this gun was manufactured. Read below for a more in depth look at this armory. Reg #21 is simply the registration number and the foundry's way of documenting and identifying each individual gun produced by their company. INSP FCH refers to the military officer responsible for inspecting the final product to make sure it meets all standards. (More on the inspector below). FDY #35 represents the word 'Foundry' and refers to the foundry's own internal control number. YR 63 is the year of manufacturing and WT 1210 refers to the weight of the cannon or bronze tube which fired projectiles. Each canon is distinct and has its own weight. More information about the specifics of this weapon can be found HERE.
The Inspectors
I did some digging to find out who these inspectors were or at least learn their name. I found a decent site HERE which lists those responsible in the 19th century for inspecting the guns and cannons at the various foundries. This cannon was inspected by FCH who Frederick Clinton Humphreys, a Major commanding the Columbus Arsenal.
Major Frederick Clinton Humphreys commanded the Arsenal and Armory in Columbus from the time of its construction in the summer of 1862 until it’s burning at the hands of Federal troops in April of 1865. Humphreys was an ironic choice to command a Confederate Arsenal in the heart of the deep south. For it had been Captain F.C. Humphreys, of the U.S. Army who commanded the United States Arsenal in Charleston, South Carolina that was seized by South Carolina state troops in late December of 1860 after South Carolina left the Union.
Official Records of the War of the Rebellion, Series I, Volume I, Chapter I
For the last eight months of the war, the Columbus arsenal was one of the major armament works of the Confederacy." Diffee Standard, Columbus Georgia in the Confederacy, (New York, The William-Frederick Press, 1954), 42. Federal cavalry force commanded by Major General James H. Wilson was approaching Columbus, Georgia, the last major industrial center in the Confederacy. A battle for the city was fought on April 16th, and on April 17th, 1865, the occupying Union forces, led by Winslow, proceeded to burn everything that could be used by the Confederate government. And so ended the life of the Confederate Arsenal, Armory, and Laboratory at Columbus, Georgia. SOURCE This source details the work Humphreys did for the Confederacy through a series of notes and other correspondences.
There are a few inscribed bronze plaques on the tube which read:
Army of Northern Virginia
1st Corps Headquarters
Lieut. General
James Longstreet
—————
Division
Major Genl. Lafayette McLaws
Major Genl. George E. Pickett
Major Genl. John B. Hood
July 1,2,3,4,5. 1863
These Headquarters were located at a
school house 900 yards westerly
Draw the Sword, with descriptive help from the NPS site, offers the following description: Erected by the Gettysburg National Parks Commission and completed in 1920-1921; consists of bronze cannon mounted on a granite base. Denotes the approximate location of General Longstreet’s headquarters which were located in the rear of Seminary Ridge. 12-pounder bronze Confederate cannon cemented into granite base, 3’2" sq. 2’7" high with muzzle up. Overall height, 10’6?. Bronze inscription tablet on west face. The inscription on the bronze plaques reads:
The Longstreet's C.S. Marker - 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 MN766.
From the Nomination Form:
1 of 4 Civil War C.S. Headquarters Markers of the War Dept. Era (1895-1927). Designates location of Longstreet's Headquarters during Battle of Gettysburg, July 2-3, 1863. Located on W side of Section 4, West Confederate Avenue, S of Millerstown Road.
Short Physical Description:
12-pounder bronze Confederate cannon cemented into granite base, 3'2" sq. 2'7" high w/ muzzle up. Overall height, 10'6". Bronze inscription tablet on N face.
Long Physical Description:
N/A
My Sources
1.
NRHP Nomination Form
2.
Virtual Gettysburg
3.
Draw the Sword
4.
Historical Marker Database
5.
Wikipedia