Gelcoat can be applied by brush, roller, or spray application. Spraying gelcoat is much more complex to do, but can give a smoother, more even finish (requiring less sanding and leveling to achieve a desired finish). The important factors to consider when opting to spray are selecting the right spray gun for the project, thinning the gelcoat, catalyzing the gelcoat properly, and deciding whether to use finishing or laminating gelcoat.
Gelcoat requires MEKP catalyst to control the working time and cure time of the gelcoat. Generally, you'll add between 1-3%, depending on the desired working time, cure time, and environmental conditions (warmer conditions require less, while cooler conditions require more catalyst).
For small touch-up applications, people commonly use cheap, disposable spray guns like Preval sprayers. Larger projects will require a proper gelcoat gun. These guns are designed specifically for use with gelcoat, based around their viscosity, spray rates, and how the product is catalyzed. Some guns require the gelcoat to be catalyzed prior to putting it into the gun, while others can blend the catalyst in as it sprays.
Spraying gelcoat also requires proper thinning, which is dependent on the gun, as well as the environmental conditions. The preferred reducer for gelcoat is styrene. Styrene is the solvent that gelcoat and other polyester resins use already. Some people use acetone to thin gelcoat for spray, but it can negative effects such as slight changes to the color of the gelcoat as it cures. There are brands such as Duratec that have gelcoat thinners that help improve the gloss and porosity of the surface.