MEK, Styrene, or Acetone. I prefer MEK as it flashes off a little more slowly than Acetone. Styrene is the best to use but it's nasty nasty stuff. Duratec makes a good product that you mix 50/50 with gelcoat to act as a thinning agent and to allow it to air cure. It works well, although, I've found that it makes white gelcoat too opaque for my liking.

DO NOT THIN TOO MUCH with the solvents. If you do, the gelcoat will not cure properly. I would thin just enough to spray...make sure your spray gun has at least a 2.3mm tip if using a spray gun. Quality of the gun is not very important...you're going to have to finish it later anyway.