Spectrum color is the best source for gel coat in small quanities. It is possible that they have the exact gray, but maybe not. However, my own experience on non skid is that it is better to use two part painted on (rolled) lineal polyurethane--either Alwgrip (Alwcraft) or Sterling. Best to put a two part epoxy primer on first, and then put glass microbreads in the first coat, apply second coat without microbeads to seal the non skid beads in place)--this is the best way to go over wearing diamond molded in non skid. We used this technique on boats which were heavily used on a daily basis. I realize that it seems like more work. But putting on gel coat on just the tops will also be difficult--you might consider thickening up the gel coat with fumed silica, and dabbing it on just the tops--that might work. If the gel coat does not contain wax, you will have to spray with PVA to get it to set properly.
Just putting on the gel coat, will fill the groves of the diamond pattern and leave a less agressive nonskid, plus the peaks will probably wear off faster than before. Gel coat is relitatively soft, the paint is harder and more durable. You can match color in each.
Remember that almost all of color coats are backed with a darker coat, to decrease the light transmission thru the fiberglass laminate, and that is why the outer gel coat layer has worn, exposing black (the occlusive color) rather than the top of the peaks being all gel coat.