Make sure the paint stripper you choose is rated for fiberglass, some very strong removers can cause damage to fiberglass doors. It should say on the label what types of substrates it can be used on; wood, metal, fiberglass. If you can't find anything strong enough without causing damage, go to your local auto paint store and ask for a fiberglass safe paint remover. Many cars have painted plastic or fiberglass pieces.
Fiberglass Front Door