there is no correct answer, but compressors and gates, wah's and things you want to process before the preamp should go there. Delays, pitch shifters, eq's and reverbs, should go in the effect chain. But everything comes down to your taste, maybe you like an octave pedal in front of the amp, maybe not etc.... who can tell but yourself?
So where to put the GT-8?
I can't say, it depends on what effect you use.