Sorry but I don't visit these forums very often. Actually ended up at this one looking for some answers for a Gamefisher 15hp of my own. Anyway, I googled your question and found 0.30 gap which is the same for my motor (see link http://www.everythingboats.com/boatbuildersforum/forum/bbs.pl?read=6097). No guarentees, but this may be the correct gap.
You should consider purchasing a manual for your motor as I did at the Sears part store since they sold gamefisher motors (http://www3.sears.com/); my motor is a mecury and yours may be as well. You'll need to enter the model# which is on the motor somewhere.