The best means I have found, short of working on it from the backside, is to use a color matched epoxy. You can find such at the West Marine or BOAT US website. Epoxy is used to repair fiberglass boat cracks. Epoxy sticks well to fiberglass.

Fiberglass showers typically have an unsaturated polyester colored gel coat (the surface you see), backed by a low glass fiber content, high calcium sulfate or alumina trihydrate content filled unsaturated polyester.

Procedure is:
1. make a small V-groove in the crack, 1/16" or so wide & deep.
2. Place a couple layers masking tape on both sides of the V-groove. Prevents epoxy running elsewhere.
3. Fill crack with "Dry Rot" epoxy. It's very low viscosity epoxy, similar to water. Keep filling crack until the epoxy gels. Let it cure per instructions.
4. Sand filled crack with 220 grit epoxy.
5. Fill sanded area with color matched epoxy.

This method should work fine if the crack was caused by impact. If the crack was caused by weight loading & flexing, then it will likely crack again, if not there, then very close to it.