I think you're on the right track. Here's what Fireworks Help has to say on the subject:
My copy of Fireworks 8 for Mac (circa CS2) has a Windows Gamma view option. I'm guessing that the Windows version had a Mac Gamma option. If the two gammas converged with the release of Mac OS 10.6, then the Windows/Mac dichotomy would no longer be applicable, so a single "Gamma 1.8" option was probably considered the best choice, in lieu of removing the menu item altogether.