Went to Antietam a few years back rather than Gettysburg but yeah, same kind of experience. Completely wiped out by early afternoon and I'm not usually someone who flags on days out. My mate who came with me thought it was just the heat but it was October so that didn't really hold up.
There's something about battlefields specifically. I've been to plenty of old locations - castles, ruins, that sort of thing - and none of them hit me the same way. My own theory is it's got something to do with the sheer concentration of traumatic energy in one place. Thousands of people dying violently in a relatively small area over just a few days. If psychic residue is a real thing then somewhere like Gettysburg is basically ground zero for it.
Would love to hear from others who've visited. Did the exhaustion come on gradually or hit you all at once? And did it clear up once you left or did it follow you home for a bit?