The idea with the MP-1 loop is to keep it at unity gain, what goes in volts wise is what come out volts wise (albeit augmented sonically by whatever is in the loop). You don't want to add or take away level but also the device in the loop needs to be driven with a good amount of signal (so you don't add noise). So set the GS21 input about half way, turn the MP-1 loop knob so you get good signal but not clipping (red) on the GSP probably have the MP-1 loop knob about the middle
, turn the GSP input up or down until you get good input signal, then adjust the GSP output so there is no volume difference when the loop is on or off.
It's trial and error until you get it right and remember as you increase the MP-1 loop send, you are turning down it's return (dual ganged opposing pot) and visa versa.
You also have to remember that the send level is affected by how "hot" the patch is, so set the loop and GSP input output levels using your loudest patch. If the GSP input leds have the occasional red flicker that's ok on you hotest/loudest patch, the GSP will have a little headroom above what the clip light is set to activate at.