Attached to the Army of the Potomac. The Corps flag is a blue swallowtail with the corps number in red.
The monument to the 1st Army Corps is west of Gettysburg on Reynolds Avenue. It was erected by the Gettysburg National Military Park Commission in 1906. (39.836105° N, 77.249227° W; Monument map: South Reynolds Avenue; Google maps for all three monuments)
Two First Corps Headquarters Markers were erected in 1913, the Doubleday marker on Reynolds Avenue (39.83558° N, 77.249578° W; Monument map: South Reynolds Avenue), and the Newton marker on Pleasonton Avenue (39.807784° N, 77.231656° W; Monument map: Pleasonton Avenue)
The markers consist of a 4.5″ Siege Rifle embedded into a stone base. This type of cannon served with the Army of the Potomac during the Gettysburg campaign in two batteries of the First Connecticut Heavy Artillery, but the big guns were not brought to Gettysburg.
The First Corps went through three commanders in the three days of the Battle of Gettysburg. John Reynolds was mortally wounded early in the fighting on July 1st. The senior division commander, Abner Doubleday, assumed command.
By all reports Doubleday did a good job in a difficult situation, but Meade had a low opinion of his abilities and replaced him with John Newton, a division commander from the Sixth Corps. Doubleday returned to his division command even though he was senior to Newton, but after Doubleday was lightly wounded on the last day of the battle he permanently left the army, with a lifelong grudge against Meade.
From the Reynolds Avenue monument:
First Division Brigadier General James S. Wadsworth
Second Division Brigadier General John C. Robinson
Third Division Brigadier General Thomas A. Rowley
Major General Abner Doubleday
Artillery Brigade Colonel Charles S. Wainwright
July 1. Arrived at Gettysburg between 10 A. M. and noon. Relieved Buford’s Cavalry and became engaged with Archer’s and Davis’s Brigades Heth’s Division Hill’s Corps. General Reynolds fell mortally wounded about 10.15 A. M. The Confederates having been reinforced from Hill’s and Ewell’s Corps made a vigorous attack at 2 P. M. with superior numbers along the entire line. At 4 P. M. the Corps retired and took positions on Culps Hill and Cemetery ridge.
July 2 & 3. Wadsworth’s Division occupied the north point of Culps Hill connecting with 12th Corps on right and Robinson’s and Rowley’s Divisions on Cemetery Ridge with detachments elsewhere.
Casualties including Corps and Division Staff: Killed 42 Officers 624 Men Wounded 262 Officers 2969 Men Captured or Missing 80 Officers 2079 Men Total 6056
From the the Doubleday monument:
Army of the Potomac
1st Corps Headquarters,
Major General Abner Doubleday,
July 1 1863
were located 230 yards s.e.
from here near the Pike
From the Newton monument:
Army of the Potomac,
1st Corps Headquarters,
Major General John Newton,
July 2 & 3, 1863