Question Can I connect a G-Major in a Peavey 5150 effect loop  

The Peavey 5150 or 5150mkII heads have hi-z effect loop (instrument level). The G-Major inputs (low impedance) require line level signal. The impedance mismatch results in an insufficient level of signal in the G-Major input generating a high floor  noise, and/or a too high level in the return of the 5150 which could cause some distortion. You will at least have to set the Input Range of the G-Major to Consumer and set the OutRange to i.e. 2dBu or 8dBu in order to get a satisfying result. You should perhaps try to insert another (hi-z input) unit in front of the G-Major as a "buffer".