FWIW, I am not a real expert, but I believe that the chemistry of gelcoat applied to a female mold
is the same basically as that sprayed on to an external surface. There is are some additives to allow the gel coat to cure a bit differently for the two different processes, but the end results are the same. And the female mold has a release sprayed on to it before the gel coat is sprayed in to allow the mold to be removed.
The advantage of spraying in to a mold is that it it comes out as smooth as the underlying mold surface is. It usually needs no further polishing as it can be as smooth as a baby's behind. Expert sprayers who thin the gelcoat just right and put it on just right can get close to a very good surface but it usually needs some significant polishing, which is an art itself. One of the key parts
of gelcoat spraying is getting the thickness right.