Quote Originally Posted by Seerow View Post
I haven't played GURPS so I could be wrong here, but as I recall was not a class based system, but was instead made with the intent you could bolt anything on to anything (sort of like the Generic Classes 3.5 variant, but more in depth). In that sense, everything was optional for a given class, but no given subsystem would be published that is going to be considered just optional.
Most of the given subsystems are just optional. Strictly speaking, the entire combat subsystem is optional, and certainly the various things you can bolt on to it (e.g. the subsystems introduced in GURPS Martial Arts) are optional. Magic is even more blatant, as there are several different magic systems that one can choose to include or not include. The system is modular from the ground up, as are several other systems.