To print PEEK you will need to install a temperature sensor capable of accurately reading temperatures that high(350C). There are probably other modifications necessary but I have not looked into it. We have not done any testing in this temperature range. Modifying the gMax to print PEEK will void the warranty.
Kyle is right about the temp sensor you'll need to use to print with PEEK and PC (PolyCarbonate) as they get above 300C and at that point you would need to use a thermocouple as your temp sensor. My first upgrade to my gMax 1.0 was to get dual E3D hot-ends for materials like Nylon and TPU (i.e. NinjaFlex); and they also sell the thermocouple sensor and wiring:
I don't know of anyone on the forum who has actually setup a thermocouple and tried printing high temp materials but I could be wrong. I just haven't bought the pieces to do my own heated glass bed for my modded gMax 1.0 but once I do the next goal is to finally install the thermocouple and try working with PolyCarbonate and eventually working toward PEEK.