Gelcoat is thick, paint is thin! Gelcoat is expensive, paint is cheaper! Gelcoat is nice, paint can be nice.
I am no expert, looking for details myself but from what I understand it is a thick gel-like resin material sprayed either into a mold before fiberglass matt is laid or on top of fiberglass matt. It contains pigment similar to paint. It must then be sanded and polished to a shine. It is pourous and brittle, thus stress cracks can appear. Paint is more flexible but starts to peel once a crack appears.