There is an excellent video series on gel coat repairs for the
fiberglass boat community. These will give you the general knowledge
you need. You can then use the group for the more specific knowledge of
work on sailplanes.

Cosmetic Gelcoat and Fiberglass Repair Volumes I-III
V I: Gelcoat 100 minutes
V II: Stress Cracks, Air Voids, and Fractures 104 minutes
V III: Color Matching 34 minutes
Bennett Marine Video 1989
Marina Del Ray, CA 90292