Just take a raincoat and umbrella. You don’t want to miss Gettysburg just because it’s raining. We were there in December years ago, with snow on the ground. Not very atmospheric for a July 2-4 battle!
Maybe you can also bring along a recording of your bagpipe-playing. If there are any ghosts to be stirred up out there, I'm sure that will do the trick.
Rain I don't mind unless it makes it warm & humid. Only under special circumstances do I travel only with an umbrella, I usually take my trusty PNW Eddie Bauer rain parka or my waterproofed trenchcoat.
But I'm also bringing the second best of my digital cameras in hopes of good photos. (The best failed and will cost a lot to have fixed -- not in the budget right now.) Juggling camera and umbrella is a nuisance.
I have three shots at Gettysburg: Sunday afternoon, Friday afternoon, and Saturday morning (assuming NnBob hasn't drunk me under the table once again Friday night *\;-).