Why do you want to take all the varnish off? Usually its enough to sand back a layer or two and then reapply.
CCScott a poster hereabouts, is the man to talk to about this as his boat is made from a couple of rainforests and requires lots of wood treatment I am sure.
I am not 100% sure but Dilunet antifoul stripper will remove varnish but will not harm gelcoat, however it would almost certainly destroy any paint as well. I think the best thing is good old fashioned elbow grease and a really sharpe blade.