No, it doesn't. So, you would hook the FX loop up to the GCX like in THIS diagram:
http://www.voodoolab.com/gcxdiagrams/01-Pedals_FXloop_AmpSw.pdf
So, on the Mark V, the FX loop would always be active, but you would be activating or deactivating it with the GCX (in the diagram example, it's...