Molded fiberglass showers are not the easiest place to install a grab bar, they have space between the shower itself and the wall frame. The gap created between shower and frame makes it not safe for attaching grab bars directly to the fiberglass, the material is thin and un-supportive of the weight displacement from when a person pulls on the bar in the event of a fall.
With any DIY project or re-modeling project sometimes you are not aware of what was built prior to the start of your project. In the case of fiberglass shower it may have some sort of reinforcing materials already in the walls to stiffen the fixture. If the reinforced fiberglass or acrylic wall is not specifically labeled as built for grab bars and meeting the ANSI load requirements, then additional reinforcing will be needed.
The problem that presents itself with adding a grab bar to a molded fiberglass shower is there is no way to access the frame to add in solid wood blocks between cavities to allow the grab bar to be mounted properly to something. This is why products like the solid mount are an ideal solution for anchoring grab bars to existing fiberglass shower walls. The Solid Mount is installed through a 2" diameter hole drilled directly through the fiberglass wall. Once secured the mounting assembly attaches directly to the stud leaving a surface to which a standard grab bars can be attached.
The key to any project is to plan, prepare, and research your workspace prior to drilling and if you are fearful of damaging the shower or wall contact a professional Grab Bar GURU+ to do it for you.