Someone else hopefully will chime in here. If your gelcoat is colored, you may want to use a primer, or sealer prior to spraying down your color. The paint shop I got my paint from told me the same thing.
As far as rattle can primer, I have no idea. I used duplicolor sandable white rattle can primer and it didn't cover very well, but still sealed ok. On the plus side, it dries wicked fast.
Although I ended up ditching the rattle can idea after semi poor results. The duplicolor paint I had dried too fast and was leaving dull areas at paint seams. So I resanded, picked up a hvlp gun from home cheapo, picked up a couple quarts and sprayed it instead. This was on a b1 though. Larger areas for sure.
Hopefully someone else will chime in for ya too.