I think you want your gelcoat to be a little thicker than that. 15-25 mils. But, on a repair of that size, if you prep properly and finish level, you can hardly avoid getting the correct thickness -- or at least the thickness of what you already have on the boat. Here is a picture that may help illustrate. Sorry it is so crude.
That is where you should be prior to sanding the new gelcoat down. The new gelcoat, if you catch it soon after it is hard, will be much softer than the old gelcoat and it will be easy to sand it flush.
For small, unobtrusive holes, you may find that marine-tex is "good enough." It may also help you get started with a white pigment base on what you are trying to accomplish.
I like using PVA to overcoat the final gelcoat layer to exclude air during cure. It's really easy to remove. But, it's something else you have to carry around/store. You can also buy a wax additive you add to the last batch of gelcaot or you can buy regular and finishing gelcoat.
The gelcoat that west marine sells kicks REALLY fast. Good for small areas; not so good for larger areas or working in the heat. You can always speed up a slower setting gelcoat with a heater and/or more catalyst.... You might get enough thickness with gelcoat paste.