Yes you can use a gel stain over your existing stain. The new coat of stain will add more color to your door. Make sure this is what you want. Undoing this stain application is a real pain.
You have a few gel stain manufactures to choose from. My favorite is Old Masters. They sell a neutral gel stain that has very little color, just the amber color. This is great for "cutting" the gel stains color for a less intense look. Plus Old Masters gel stain is easier to use, in my opinion.
Preparing the door is very easy. All you need is a clean surface to start with. Carefully wipe the door with a damp soft cloth. Don't push real hard as this could scratch the existing stain and you want it in good shape for a better final look. Once the door is clean and dry you can go ahead and apply the stain. Wait at least 24 hours before applying the clear coat, I recommend two coats of clear.
For information check out <a href="https://www.house-painting-info.com/staining-a-fiberglass-door.html">Staining a Fiberglass Door</a>.