I'm using my G-Major with factory settings for the Global output. I've got the front panel input and output controls set at around 2 o'clock. If you're experiencing a dramatic drop in level when the G-Major is in the loop you might check your line level settings. Also, some effects types (compression, envelope filter) / mixes will tend to limit or reduce the peak output through the loop.
When I first bought my G-Major I used it in the effects loop of my Fender Concert II combo. Using a simple reverb effect on the G-Major, I would play without the G-Major plugged into the fx send on the combo, then with it plugged in until I had a consistent volume level between the two.
Also, the bypass on the G-Major does not bypass it's input and output level controls - just the effects blocks. So it's not a good way to A/B.
The output level on the TriAxis determines the effects return level (there's not specific control of the send level other than the patch master level). I'm running my TA's output at 2 - 3, and my 20/20 between the 8 o'clock and 12 o'clock positions depending on how LOUD I'm playing.
You might want to reset the G-Major to factory settings, then re-structure your gain by using the TA output level and power amp levels to set the general volume level, run your TriAxis with the effects loop on and choose the appropriate effect (or NON-effect - you can create a no-effect effect or a simple minimal reverb) to use for that program / patch. You'll always go through the G-Major but your levels will be more consistent.
jbw