There are four major international airports within two hours of Gettysburg, and one fair-sized regional airport in Harrisburg.  Both of the Washington, DC airports (Reagan and Dulles) are within 1 and 1/2 hours of Gettysburg and there are plenty of flights in and out each day and plenty of options to get you from there to Gettysburg.  In the end, the best way to really see Gettysburg is to rent a car once you land, it provides you so much more freedom to see the area (the battlefields, apple orchards, Amish farms, etc).  The airport in Philadelphia is about two hours from Gettysburg, but not as convenient as the DC airports due to having to take the PA turnpike and dealing with that mess.  By far, the most convenient airport  to and from Gettysburg in terms of the amount of flight options in and out, the amount of time it takes getting to and from Gettysburg, and with the least amount of traffic to deal with is Baltimore-Washington.  You can easily get from BWI to Gettysburg in 75 minutes and all the major carriers fly in there.  You can fly into Harrisburg as well, it is close being only 45 minutes away, but the selection of flights in is very small which can often result in some expensive flights.  If you're not into flying, you can always look into taking the Amtrak train to Harrisburg or York and you'll find yourself within 45 minutes of Gettysburg.  Getting to Gettysburg by car is very easy and there are several major interstate highways within 45 miles, including I-70 and I-76.

Please find some great information on Gettysburg, PA here.