In '68 the smallest V8 in the Mustang was the 289cid. It was a C-code. Best bet is to check the VIN; there are numerous online sources that can help you decode the VIN. The engine code will be the 5th letter (or possibly number) in the VIN. The first alphanumeric of the VIN should be a number and will be the year in the decade the car was built. Therefore the VIN should start with an eight (8).
sonicpony03 pretty much hit the nail on the head, but to reiterate; 1967 was the only year Ford used the GTA badging, which just meant it was the GT model with an Automatic transmission. The GTA badge spanned all three body types: coupes, convertibles, and the fastback. This was for the 289 and 390cid although I'm not sure if it included each engine. Meaning there was a 2V 289cid and 4V 289cid (HiPo?) and a 4V 390cid and that I'm not sure if it included both 289cid versions. (Actually there were three 289cid versions, a third 4V had a bump in horsepower and placed into some Shelbys).
For your 1968 TheMustangSource.com | Timeline: 1968 Mustang
For 1967 (and the GTA badge info) TheMustangSource.com | Timeline: 1967 Mustang
By the way someone around here has the new S197 body style with the GTA badge on the fenders and it looks really good.