The best one of those silicone filler materials (microspheres) that I've found is called cab-o-sil. The only problem is that it's sometimes harder than the gelcoat so you have to be careful when you sand it not to sand into the surrounding gelcoat. There are some pre-mixed repair putties out there that may be called mold repair putty. One company is Hawkeye Ind. Here's a link:
http://www.duratec1.com/ag07.html