I had to strip a too dark finish off a fiberglass door using a gentle stripper, cleaning with lacquer thinner, sanded lightly with 220 paper and restaining with artist oils for a richer finish. The inside held up beautiful but the exterior peeled badly after 2 years, one door is under a covered roof and only gets sun in the early morning. I won’t be able to stain it to match interior using a gel stain. Was it the artist oils?
Probably the artist oils didn't stick as well as a gel stain. You didn't mention putting a clear finish over the artist oils. If no clear was used then this might be the problem.
All stains, gel or not, need protecting with a good clear finish. Generally, quick fading and possible peeling takes place if not protected with a clear finish. Any good clear finish can be used as long as it is rated for exterior use. Example; exterior spar varnish, polyurethane or acrylic urethane.