ProGrid: How do mixing consoles control the mic gain on ProGrid units? Can they use their own faders / knobs or do they have to use the ProGrid software?

In the ProGrid software set Emulation Mode(also called Special Mode) for the device connected to the console(PG2-MADI-xx or YG2 or Y3R) 
This allows the ProGrid device to send and receive command controls from the console stagebox / desk - in the case of Yamaha it's AD8HR,Soundraft/Studer - D21m frame, Lawo - Dallis Frame, etc. 
Those commands aretranslated into ProGrid protocol allowing the Console operator to control ProGrid preamps directly form the console.Currently Special Mode is available for Yamaha, Soundcraft, Studer, SSL andLawo.