In most cases, the gel can be sprayed directly to repairs, gel coat, etc. It sprays fairly thick, nothing like water borne or enamel paint. In all honesty, I would use the least amount of primer possible.
The last gel job we did, 320 was used to block the entire boat and I used Eastwood Black guide coat for the blocking, using a long board and a flex block.
We did the entire deck of a Spectra 20 removing the wood grain and bow rail. All the holes were glassed in then topcoat was applied which was just tinted surfboard resin.
All the repairs came out better than the rest of the boat.
We were using Duratec at the time.
GT