Hey Bazza, welcome to the depot
. There's a bunch of ways you can patch things, all with different results. The GT100 isn't a great fit for a MP1 rig as it wants to rule the world and seems largely designed for recording using its amp simulators etc. How you patch it depends what you want to do eg do you want to record with it ?, do you want to make a live rig (do you have a power amp and cabs ?).
Probably the easiest way is how RG uses his MP1 with his boss GT5 thingy (in essence, it's a similar idea/unit to the GT100) and put the MP1 in the GT100 Fx loop. So your MP1 becomes a glorified distortion pedal. This will work, but the MP1 is then mono but you can use all the GT100 features.Alternately you could put the GT100 in the MP1's Fx loop and turn off all the stuff in it you don't want ie all it's preamp stuff.Either of these options will work but getting the gain structure right will be a bit tricky as neither of them are designed to work this way particularly as both units expect a guitar level signal on their input and output a line level signal.
I'm in Canberra so I'll PM you my phone number, your welcome to give me a call and we can talk it through (if you'd like)