Is the crack just in the gelcoat or has the FG laminate itself cracked? If it is just a surface crack you can us any polyester or epoxy resins like a West Marine or even JBweld. If the laminate itself has failed you will need to reinforce it with FG or screen to restore the structural integrity of the laminate. I would also put several headers underneath the pan for added support. Unless you are very good at FG repair it will not look pretty so I would then add those non slip stick on strips that people add to their bath tub floors to cover the repair.