Hi,
That looks like it needs some glass cloth, not just filler. When you do glass it, West Sytem is fine, but so is regular polyester resin. If you do plan to gelcoat over an epoxy repair, don't use anything other than West, as gelcoat will not stick to most other epoxies. It will adhere fine to polyester. You will most likely need some "bondo" fairing putty between the glass step and the gelcoat. I find Evercoat Formula 27 to be great.
The best gelcoat around is from Minicraft in Florida. They sell it with wax added so it will cure in air. They also sell reducer so you can spray it. (DON'T use acetone to thin if spraying). They also sell tints if you want to do an exact color match (which I have done very often very sucessfully, using all their products). They also sell a good faring putty called "A-Putty". Slightly better than the Evercoat formula 27 stuff, but more expensive. When done, wet sand with 400 grit and then buff with 3M Finesse-It and a high-speed buffer.

As far as how to do the repair - consult the West System book. http://www.westsystem.com/

Good luck!

Steve
Hobie Fx-1, Sail #211