I'm currently going this method also.
I was pretty much settled on doing gelcoat over old gelcoat and I decided otherwise when I started thinking about how much work would be involved, and then it would only be as good as the old gelcoat below.
So, for now, I'm going to do oil-based paint over the old gelcoat.
I roughed it up with 40 grit. I'm not looking for pretty because it's just the floor of my boat, and I'll be putting sand in the paint for non-slip.
I just want it to stick good and reasonably resist foot traffic. I did use an oil-based primer from Sherwin-Williams, but I may find out I shouldn't have, and instead just painted right over the sanded and cleaned gelcoat - time will tell. I already have some Rust-Oleum Marine Topside paint that I'll be putting over that tomorrow afternoon. I'll put Majic Hardner in it to the instructed ratio.
I'll keep an eye on this thread to see how things go for you.