Welcome! Need support, you got it. Or share you ideas and experiences.
If it ran correctly before you changed the plugs, perhaps you gapped them incorrectly or used wrong type spark plugs. If you don't know if it ran correctly before you replaced the plugs it could be many things such as dirty gas filter, pinched fuel line, closed gas tank breather screw, improper gas/oil mixture, gas/oil mixture issue (stale/contaminated), worn breaker points and/or condenser (under flywheel on my motor), a carburetor problem (if your engine uses a carb, there's a screw for rich/lean adjustment, or carb might be dirty or just plain broken), coil issue or worn plug wires. Eliminate the easy suspect items first. I'm not a boat motor expert, but these are some ideas.
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.
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