Apologies for this very late reply. I don't think I got notified and I haven't been super active on here lately.
1) I did not use gelcoat I used 2 part epoxy. Generally epoxy and UV is a bad combo but this specific product I selected and recommended is UV resistant and its formulated for painting like the sides of industrial water tanks that are out in the sun continously. After 3 years it has not chipped, chalked or yellowed in the exposed areas.
2) This epoxy product recommended the use of their primer and it was not expensive. The flooring was very porous and it took several coats of primer before it started to really fill in and close up the pores. If you use a different product primer might not be needed but I definitely did not want the coating to start peeling up and the gatorstep come up with it after a few years. Again after 3 years it still looks like new.
3) bondoglass worked gear for me. It was available locally and was easy to work with.