What gearing does my 88 Gt have?

Either 3.27s or 3.55s, but it's hard to tell because we don't know your tire size or how well your speedometer is calibrated.

Count the number of driveshaft rotations like Mustang5L5 says, but I suggest turning the wheels 10 times (have a helper turn the wheels, while you watch the driveshaft). I can tell you from experience it's hard to tell 3.27 rotations from 3.08, and 3.55 from 3.73. 35.5 vs. 37.3 is much easier.

Or you can remove the differential cover and start counting gear teeth. You probably need to change differential fluid anyway.