I was there just last week and ate at a place called The Pub and Restaurant and also one called The Blue Parrot Bistro. Both were good, but nothing spectacular. Both are also right in the center of downtown and there are a number of other restaurants in the area.
You may want to do a Gettysburg keyword search because there have been prior discussions about restaurants in the area. I would add Cafe Saint Amand to your list. It's a decent French style bistro on Baltimore Street. It's BYOB, in case you want wine with your meal. The other possibility would be the Cashtown Inn, a historic place about 15-20 minutes from downtown Gettysburg. Though I have had some issues with consistency, there is something very relaxing about a meal in the tavern room next to the fireplace. The menu is reasonably interesting and the wine list is extensive. But you should check first before driving because, for example, they are currently closed until this coming weekend.
I second Cafe Saint Amand - had a nice dinner there. The atmosphere isn't totally bistro-like, though, if that matters. Just my opinion; but the food and service are good.
Cashtown Inn is not at all bistro-like, but I've had some very good meals there - plus the atmosphere is nice, and the staff are efficient while making you comfortable.
Have also had some good meals at The Inn at Herr Ridge - food is excellent and locally sourced, though I wouldn't exactly call it bistro-like. Service is leisurely, which isn't an issue for me, but may be for others. http://www.innatherrridge.com/dinner.pdf
I don't know if this thread will help, but just in case - http://chowhound.chow.com/topics/743296