don't want to sound bossy ere but... you might be safer find the source of the leak & fix it, before you do in the head gasket. if you find the loss of water very small and cant pin point a leak, then maybe the head gasket could already be jiggered.
by all means try the radweld first. when i used to fiddle with heavy plant machinery a small leak could be easily cured by cracking an egg into the radiator
but i wouldn't recommend that with a car as it could partly block up the interior heater matrix.