I use spray guns at times in my work, although never with polyester. I did find a void in the centerboard trunk cover on my Picnic Cat which I was able to fill and blend very nicely.
Not sure how big an area you're talking, but an airbrush with the largest nozzle was suitable for what I was doing. After filling and leveling the void with epoxy and glass to 220G, I thinned the gel coat I got from Com-Pac to a sprayable consistency. I don't recall how much styrene I used, but it was in excess of what was recommended. Test everything off the boat first to be sure you're OK. After masking, I sprayed on enough to cover the fill, let it set up, and sanded it down with 400G. I needed to spray a bit more where it was thin, I probably did that the next day. Leveled with 400G again, and rubbed it out with a maroon ScotchBrite pad and Novus #3. Couldn't make it out, even in person.
Taping to a hard edge is best, of course, but in my case I feathered out the blend fore and aft. The sanding took care of any dry spray at the margins. I used a good airbrush, that I didn't want to trash, and made sure I broke it down and cleaned it thoroughly before the polyester set. No bad surprises. I might not use my best Iwata gun, but I also wouldn't want to have to deal with a bad spray up from a cheap gun.
My boat dry sails out of my garage, so the gelcoat was in great condition. The gelcoat Com-Pc provided was a little light in color, but with time and grime it matches perfectly. An oxidized gelcoat won't be as easy, especially in a dark color. Good luck!