I am installingf a shower stall and not sure how to make the trasinion from fiberglass stall to sheetrock wall

You should install at least greenboard, not drywall or sheetrock, all the way to the lip of the shower pan. Most stalls are held in with construction adhesive and you need to good surface to bond to. The greenboard goes all the way to the ceiling. The top edge of your shower stall should round over and terminate at about 6 1/2 ft.

I am installingf a shower stall and not sure how to make the trasinion from fiberglass stall to sheetrock wall
If this is new construction, and you are installing a fiberglass one piece stall with build-in pan, you will not need sheetrock or greenboard. Setting the stall in rough framing needs blocking installed first for backing nailers for screwing in the stall lips direct to the framing. Make sure your plumbing stubouts are well secured and that the stall is accurately lined up and predrilled for aligning the stall in place. Access for installing the diverter control valve should be taken into account for the final installation clearances.

If this is a remodel of existing sheetrock walls, removing and shimming out the walls to match the size stall is recommended first. Then proceed as above. The drain has to be set up where the pan section sits solid onto the subfloor surface. There are different methods of supporting the pan to keep it from oil-canning. A secure waste flange does not support the pan. Use a concrete quickset placed down on the floor surface before dropping and extruding the fill outward. This makes a solid base on a cutout slab configuration. If the base is mounted on an OSB or ply subfloor then a foam filler applied later will shore up the base for a solid feel. Most modern remod units come in two section molds to allow easy assembly of the base first. Follow the manufacturers instructions and things should go in pretty easy.

After the stall is in place and screw secured around the lips, then a final Ferro board paneling can be put in place after the ceiling sheathing has been installed initially.