The first part of staining a fiberglass door is deciding if you will stain and finish it in place, hanging on the frame, or off on some saw horses. Personally I like finishing these doors hanging.
First, remove the weather stripping from the door frame and apply blue painters tape to any areas you don't plan on staining, glass or other non-removable accessories. Now remove the door knob and protect the floor with a drop cloth or cardboard.
Now clean the door with mild soap and water to remove greasy finger prints and dust. You could wipe the door down with denatured alcohol and a clean rag instead. This method dries within seconds and does a good job.
How you apply the stain depends on your skill and the stain. I haven't used Olympic gel stain, but have noticed that many gel stains are similar to each other. I like the brush method instead of using a rag.
After the stain has dried completely, minimum of 24 hours, the clear protective finish coat can be applied.
I have written a tutorial about <a href="https://www.house-painting-info.com/staining-a-fiberglass-door.html">staining a fiberglass door</a>, please check it out.