Do you mean like individual rocks and trees? Leftover bullets or other equipment, or buildings and fences and such that were standing in 1863?

Uhh, yes. Lots of things. the town of Gettysburg itself still has numerous buildings and structures existing from the battle, as well as some on the field itself. Many of the stone walls have been repaired or built in the modern period, but there are some "witness trees" still on the field tagged with a little metal disk. As for relics, there is no way to know how much stuff is still all over the place. This is true with virtually every semi-preserved Civil War Battlefield. With tens of thousands of people carrying, running, dropping, losing things, battlefield initially are littered with objects. Many of the items in Gettysburg's museum were picked up directly from the field. It is for this reason that the National Park Service has a standing prohibition on metal detectors in the park. Relic hunting on Federal ground is very illegal. But it is necessary because there really is that much stuff still buried.