Use the right type of gelcoat

There are two types of gelcoats, with or without wax. Wax is used on gelcoat to seal the gelcoat as it curates. It floats to the surface and creates a sealing layer that allows the gelcoat to cure. Gelcoat is a very thick paint, so it tends to quickly separate and it needs to be well mixed before it's used. The wax adds to the tendency to separate, specially at temperatures below 55 degrees Fahrenheit (it solidifies and clogs the sprayer). Professional crews use a shaker and a heater to keep their gelcoat around 70 degrees and to make sure it's throughly mixed before they spray it. The temperature and shaker action keeps the gelcoat from separating and keeps the wax melted. However, for a DIYer, without a way to maintain the temperature or shake the gelcoat to submission makes it very hard to use gelcoats with wax. It will laminate. So, we suggest using a Laminating (non-wax) gelcoat.

Make sure your gelcoat color matches

If you want to be certain the color of your repair matches your boat, you can call your manufacturer and buy the gelcoat from them. They usually will ask for your boat's ISAF number and the year it was made. It's definitively more expensive than using generic gelcoat, but it may be worth the extra money if you want a 'like new' repair.

Reduce the gelcoat viscosity to fit your sprayer

To reduce the gelcoat to be thin enough to come through the sprayer nozzle, you will need to reduce it. To do so, you will need to use a thinner or additive to reduce the viscosity. There are two main types of additives.

‍Reactive. They become part of the gelcoat. So, then the gelcoat pigment is reduced (e.g. 50% on a 50/50 mix), it becomes more translucent (looses pigment) so you need to apply more layers to get the same finish. It also does not last as well since it becomes translucent as the layers ware. Also, most of this products are not designed to be used under the water. As a result, they can blister and the only way to fix it is to sand it down and re-apply.‍
Non-reactive. They evaporate as the gelcoat dries. Therefore, they don't reduce the amount of pigment and have no problem being used below the water line. So, folks with experience using Gelcoat will only reduce with non-reactive thinners. However, they both can change the color of your Gelcoat as it dries, to it's recommended that you test it first. The two most common non-reactive thinners are: Styrine - has less of a chance of changing the color and Acetone - has more of a tendency to change the finish color.

Select your sprayer

If you have an actual sprayer you probably know how to check for viscosity and prepare the gelcoat for your sprayer. Otherwise, you can use a disposable sprayer, Preval makes a good sprayer that you are likely to find at your local Home Depot or Amazon.

Try out your gelcoat and sprayer before you start

From experience, make sure your gelcoat and sprayer are working well before you spray the boat. Keep in mind that you will need a larger diameter sprayer to work with gel coat or resin and it will help if it includes a warmer to keep the wax as liquid as possible. It will take a few tries to get your proportions right, be patient.The most likely issues you will have are the following:

The gelcoat will not dry. Reducing the gelcoat affects how the gelcoat dries. Make sure to spray some gelcoat on a piece of cardboard to the thickness you need and let it dry. If it's not dry within an hour, it's unlikely to dry at all. You will need to remove all the gelcoat with a rag and some acetone. If your gelcoat is not drying, you likely need more hardener.
The sprayer will get clogged. it's also possible that the sprayer will get blocked or spray unevenly and with large 'chunks'. If so, your gelcoat is likely to need more thinner, or it has too much wax.

Other references

Spraying Gelcoat DIY Style for Boat Repairs
How to check viscosity for a spray gun
Gelcoat Repair Primer by Fred Schroth. This article tends to appear and disappear, so the link is to the Wayback Machine.