Are you trying to match the gelcoat with paint or more gelcoat?
Any time I've had to match gelcoat, e.g. to repair a small chip, I've used one of the kits where you can mix color into the gelcoat until it matches. So, I start out with the white and adjust it very, very slowly until it gets a good match. I look for excellent sunlight. Then I go from there. It is never perfect. But, since I've only every done small bits, it has worked well and no one but me would probably ever notice.
If you looking for a paint color that is a good approximation, each of the major brands has a color chart. Stores that sell the paint usually have it. Or, it can be ordered from an online distributor and delivered by mail. From there, I like to cut the little squares out with a utility knife and find a good match. I've never tried color-mixing or tinting any 2-part marine paint myself.
Make sure the area you are trying to match is really clean first. A light polish (or wet sand), but not wax on the spot is a good idea. Don't wax because it can interfere with adhesion if you don't get it all off during your prep.
At any rate, that's been my approach.