No need to remove the gelcoat for body filler. Just key the surface thoroughly so the filler gets a good mechanical grip. I sand the area with 80 grit and that seems to work.
The new fillers seem to be much more flexible than the old bog. I'm using a 3M brand body filler and it seems pretty good.
Automotive Aftermarkets Filler, Adhesives, Coatings & Sealants:*3M
Cheers