On a return trip, and first time flying out of KGKT, called to pick up clearance. It was extreme VMC. Airport was super busy, as was Knoxville airspace, and a had a line forming behind me while waiting for clearance. Do you guys typically just depart VFR, and pick up in the air? Seems like that would have been smarter. Also, the A/FD gave a phone number to call for CD, but calling they just had me call up on departure freq. How would I have known to use that frequency if I had wanted to pick up my clearance in the air?