you can use an HVLP gun for gelcoat. I do frequently. Just be sure your tip size is 2.3mm or larger. Northern Tool has a decent $30 gun with the larger tip (the green gun). There's no point in spending a ton of money on the gun...it's going to end up a little lumpy anyway.

I just bought a cup gun that has a 6mm tip (good lawd). I haven't had a chance to use it yet but am hoping to make a cradle mold with it in the next couple of weeks. The cup gun spits out unthinned gelcoat but is, in my opinion, probably not good for finish work (perhaps if you have a finer tip). It is great, however, for interior mold gelcoat prior to lamination and looks to be super easy to clean up.