How are you using your G-Major, in front of the amp, in the loop?
The first thing is to make sure the amp and the unit are using the optimal input/output volume levels.
Also note that if you're using the unit in your loop and playing mostly at a very low volume, it will sound 'pants'. The Mark V fx loop generally does, at low volumes - because there will be some signal 'bleed through', which can make the amp sound like it had a bad, out-of-phase parallel loop (even though it is serial). If this is your problem, the solution is simple: turn your amp louder.