Hey VoxG2, short answer is the GP-8 is a mono in - stereo out device and you need a stereo in/out Fx unit to do what you want. The GP-8 may have front and rear inputs, but either one will turn off the other (most likely) or, using both inputs, the ADA A & B signals will get mixed together in the GP-8 back to mono (highly unlikely). If you like your current sound (as you say you do), just loose the front input (I suspect it will make no difference, the GP-8 isn't a stereo input device).
To run properly in stereo (both MP1 and GP-8) you need a mixer, eg MP1 A/B ch 1&2 (panned L/R); Fx send1 (or feed it from the MP1 Fx loop send) to GP-8 in (front or back?), GP-8 out A/B ch 3&4 (dry/Fx Mix set 100% on GP-8).
So currently, what you have is one channel of the MP1 going into the GP-8 (I know you plug in MP1 A&B to the GP-8 front and back but one will cancel/turn off the other, and then, the GP-8 going stereo into the B200s, so, in the end, what you miss is the MP1 stereo chorus section.