but know that one cannot sand a fiberglass door.

Why not?

I really am not sure why not but I've been searching for information on this subject and that seems to be a common aassertation. I thought that maybe it has to do with the grain just being on the surface but if anyone knows for sure I would love more info.

I have sanded many fiberglass doors, not a problem.
Lighty hand sand, you don't want to take a power sander to it.

The directions for Therma Tru say that the doors should not be sanded, but I believe this refers to sanding the raw fiberglass skin, not really the finished door. If you sand the raw fiberglass you can either reduce the profile of the grain pattern or you can scratch the fiberglass, both of which may show up when you stain the door. You would need to sand pretty hard to do this though. Lightly sanding and sanding the finish that is already on the door should be fine and this is something you need to do for sure if you are painting over a stained finish.

This makes sense. I will lightly sand the door. Do you think the gel stain will adhere to the door if lightly sanded? The gel stain will be on the inside of the door and I am planning to paint the outside.

The stain will stick just fine. You should apply a clear finish of some kind though over the stain after it is dry.