For Parking at Gary Pirkle you want to use the Upper Parking lot which is off of Suwanee Dam Road.  Do not go to the entrance on Austin Garner.

If you are coming from 20 on Suwanee Dam, go past Austin Garner, and the entrance will be on your right.
If you are coming down Austin Garner from Riverside, go past the entrance to the stop Sign(Suwanee Dam Road).  Turn Right.  Entrance will be on your right.