Normal filler can be used ok as long as its flexible and the same rules apply to fibreglass as to metal bodies, filler is for small imperfections not filling holes or building up large depth profiles.
If you do have a large sunken area a couple of tips that might help
A. Get some heat on the sunken area and apply constant pressure from behind with a prop of some sort, quote often you can get the fibreglass to move. If you then keep it propped from behind whilst it cools it should stay in the new shape.
B. if you do need to raise the profile of a panel consider grinding the gel back and building it up with layers of fibreglass then finish with bridging filler and final finish with a flexible filler just to smooth the surface if required.
When painting fibreglass i always use and etch primer first, opinions differ on this point but it's always seen me alright. Normally just a single coat of etch then straight onto a couple of high build for wet flatting down before top coat.
Another tip is to get the body into a heated booth and get it warm, or you can use a heat gun with care. This will reveal any voids in the fibreglass as the air trapped in them will expand and show as a blister when heated, nothing for it but to dig them out unless you want your car to develop a lumpy body on the couple of days a year it gets warm enough :-)
HTH
Mac
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