Luggage and bags:
A backpack is a must for a day or evening in the military park - try walking too far from your car carrying your water bottle, camera, sunglasses, flashlight, cell phone,...I made that mistake only once!
Clothing/Shoes/Weather Gear: If you plan to do a lot of exploring off the beaten paths, good hiking boots are a must - lots of hilly, rocky terraine through the wooded areas of the park. Also, a rain parka is a good idea, but the plastic ones are sold almost everywhere for just a few bucks.
Toiletries and Medical Supplies: Sunscreen! Sunscreen! Sunscreen! Also, antibacterial hand wipes and bug repellent in warmer weather.
Photo Equipment: Be warned - camera equipment is known to malfunction and batteries drain at certain areas of the battlefield. Some claim it's the doing of the ghosts of soldiers...oooooooh........
Miscellaneous: Make sure to bring plenty of water with you. No concessions are sold within the park and while stores & fast food restaurants lie just outside the park's entrances, who wants to pack up and leave for a beverage run? Also, although public restrooms can be found at several locations within the park, they are closed for part of the year - something parents with youngsters will want to keep in mind.
If you are visiting Gettysburg in the spring/summer and you plan on visiting the battlefield (which is the reason most come here in the first place!) BRING INSECT REPELLENT AND MAKE SURE IT CONTAINS DEET!!! The ticks are EVERYWHERE here, and they are sneaky little b***ards!
Other things to consider....if you're taking pictures, or using anything electronic, having a supply of EXTRA BATTERIES is a very good idea. I take lots of photos here and I can tell you there is a lot of truth to the rumors of batteries being drained. Although I have more problems with that in the daylight hours.
Whether the energy "drainage" is due to the paranormal, or just the fact that underneath Gettysburg are huge deposits of various rock "types" containing lots of quartz, or the combo of both, I'll leave that up to you to decide, as that lies in the "opinion spectrum".
But I'd say the battery drainage rumor is FACT, because I never go through batteries anywhere else like I do here