Easy option: white marine tex, like mobjack22 mentioned.
Slightly more difficult option:
- Use a countersink or dremel to enlarge/clean/bevel the hole.
- Clean everything really well with acetone.
- Put a piece of clear packing tape behind the hole to prevent leaks.
- Mix your gelcoat and mek. Don't add cabosil or other thickeners because they can cause discoloration.
- Add drops of mixed gelcoat into the hole until it bulges. Do a drop at a time so you don't get air bubbles.
- Add another piece of packing tape over the top.
- When cured, pull the tape and sand by hand. Work your way from coarse to fine grits. Something like 60, 150, 220, 320, 600, 1000, 1200. Optionally follow with compound and/or polish.
Depending on the temperature, you can do a few screw holes with 10-15 minutes of effort in an hour of elapsed time. Using a gelcoat patch kit you should be able to get an almost perfect match. Just get the color you want before adding the mek.
These aren't the best pictures, but you can get an idea of the expected results.
To the left side of the vhf you can see some screw holes and torn gelcoat from where I removed an old panel.
This is after repairing with a gelcoat patch kit. The color is a near perfect match.