We have a outside fiber-glass door and painted with Gel Stain brought from Home Depot.
But, the Gel has been peeled off after the summer. Any advice?

Gel stain was likely applied too thick for fiberglass. Two very thin coats should have worked. You should strip any remaining stain using a gel stripper. Wash well and then use a latex prim,er followed by an acrylic latex top coat. I add Floetrol to help the paint to smooth out. You can roll with a very short nap roller but avoid a foam roller, they create tiny bubbles in the paint that might stay in the finish.

Gel stain was likely applied too thick for fiberglass. Two very thin coats should have worked. You should strip any remaining stain using a gel stripper. Wash well and then use a latex prim,er followed by an acrylic latex top coat. I add Floetrol to help the paint to smooth out. You can roll with a very short nap roller but avoid a foam roller, they create tiny bubbles in the paint that might stay in the finish.

I can agree with ballengerb1. After stripping and cleaning, a couple other ways to get this done is:
1) Apply the paint with a power sprayer such as Wagner. This eliminates all bubbles and nap marks. Use the paint type listed above. These can be rented or purchased for under $100. Practice before doing the door!
2) Take the door slab to a local automotive body shop. Bring a color sample. These places will paint the door extremely well. The cost can be negotiated but should run $50-150.

Is that the link you really want?

Very accurate