Use the loop. That way you get one more tube stage before the signal reaches the power amp. It will work the other way, but you have to be careful. You should be very careful about sending the Triaxis outputs to the G Major because they can be very "hot". Keep the output low.
If you use the FX loop you don't have to worry about sending a too hot signal into the G Major.
Alternatively, you could try sending from the loop, then into the G Major, then directly to your power amp. Some people prefer this. It becomes a tonal choice. Personally, I'm 100% with the loop because it is easier to deal with and sounds better to me with the extra tube gain stage (V5).