The key when working with Gelcoat is to make ssure that you dont have any shine left. If you do there may still be some mold release agents still on the surface and even though it may stick now it will release later which would suck. I agree that breaking the surface with 60-80 grit paper is the right thing to do. Also bondo or fillers will adhere "well" to Gelcoat however if you really want to do a real good job grind out with a Dremmel the cavity area to get the Gelcoat shine out and apply Gelcoat to that area. One note however, if the depth of the groove is greater than an 1/8" then you should fill the area with a mixture of resin and fiberglass fiber. You can tear off strands from some fiberglass mat, mix it with resin and fill the area, messy but once you do it you will find the trick. Let set and harden, smooth and grind then apply the gelcoat to that area. Some may say use bondo or equivalent (as long as it has very good elastic properties) at that point and that's fine. key is to make sure that any filling of crevices are as strong or stronger than the original laminate.