Unless this soldier became a casualty at Gettysburg (killed, wounded or captured), you probably won't have real proof that he was there. However, if his military service file records him as present on June 30, 1863, it's a very safe bet that he saw action on July 2, 1863. Company commanders produced these rolls roughly forty-eight hours before the battle, and unless a soldier became very sick or left his company during that short period of time, he was present on July 2nd.
Do you have this man's military service file?