So I just got back from Gettysburg. 10/10 would recommend. The battlefield is really cool, but if you want to see all of it I would plan to stay for at least three days.
Here's my understanding of how the battle played out.
June 29-July 1, 1863
July 1 - fighting begins
93,000 Union troops VS 73,000 Confederate troops.
Circle - canon
Line - tree
Square - Union
Triangle - Confederate
The battlefield was about three miles long.
The Confederate held the high ground in the North and Northeast. The Union was forced to advance and defend in the open fields. The Union suffered significant losses and was forced to retreat.
July 2&3, 1863
The Confederates and the Union had moved the battle East. The Confederates had a little high ground and came from the trees in the East. The Union was defending on the opposite side of the fields. The battlefield was roughly three miles long. It was a bloodbath. About half of all soldiers on both sides died charging and firing at each other in the open fields. It ultimately ended in the Union's favor.
If you can go to Gettysburg I would highly recommend you do. Plus the town is really nice. Just make sure to practice your parallel parking skills and you should be fine.
Here's something I drew on the ride home: