It very well could be the head gasket if you're sure that it's not leaking from elsewhere. Did you check the allenhead caps on the sides of the motor? They're like bolts, but they don't have a head on them, just recesses for a large allenhead socket/wrench. One of these started leaking on me right after I swapped it, but it was leaking oil. Took me a few days to figure it out, but I was glad that was the problem.

It could also be your coolant channel on the back of the motor. It's a metal channel that's usually black and it connects from your water pump housing to your thermastat housing. There are o-rings in there that can go bad and leak. There are also a couple hoses that run from it that could cause problems. Check those out and see if that might be your problem.

hih