I plan on spraying gel on cabinets and bulkheads but am unsure how much glass needs to be on the wood to keep it from cracking. I have some 3oz surfacing cloth will that be enough?

3 oz. cloth would be way too light. You would want 1 1/2 oz. mat next to the substrate assuming that it is 3/4" plywood or similar. Then at least two layers of 10 oz. cloth with peel ply on top for a smoother finish. Vinylester resin would be suggested. I would add Duratec to the gel coat to help it flow into a smoother finish with minimal sanding.

Jesus he is covering a cabinet not building stringers.

what do you mean cracking?

i do agree with th peel ply for making a smoother finish.

Thanks for the response! I expect that the gelcoat will stress crack after a wile just like the deck of a boat does; if its not laid up somehow. I just dont know how carried away to get. I dont want to spend a week shooting gel, sanding and buffing just to have it all stress crack the first time I get crazy. So a layer of 1708 should do it on 1/2 plywood? Where is a cheep source for peel ply? l I'm starting to see why they used carpet, and glue on plastic panels.