Bondo will stick to fiberglass, but it WILL crack back out in time. I work on fiberglass everyday and it will sand easy.
1st you need to sand the area with some 80 grit sand paper. Then get some resin and mat (fiberglass). Mix the resin according to the directions, (making sure that your in the shade) and place a piece of the mat over the area. NOTE If you tear the mat so there is not a cut edge it will almost disappear when applied.
Apply the resin with a chip brush (the cheaper the better) and wet out the mat until it is no longer white.
Apply as many layers over and around as needed to fix the damaged area.
Then allow it to get hard.
Once hard you can now come back and sand it smooth (220 grit) ( or start with 80 grit if you got it too high and have a lot to take off and then go to the 220 grit.)
Then if you still don't have it as smooth as you would like it, take some of the resin and mix in some sort of powder. Talcum powder works just fine. Mix it to a paste the consistency of bondo and harden it and stir it well and apply like bondo then once that hardens you can sand that smooth and you will have a repair that will last much longer.