I have a preamp with stereo fx loop, but it has only one send jack and two returns (L and R). For a good stereo use, can i connect send to IN(L) mono, or do I need a Y cable to connect the mono send split to IN(L) and IN(R) ?
Just use the Left input on the G-Major 2 (mono) and use the stereo output.
The input in G-major 2 is summing left and right inputs to create the effect . The effect engine is mono in and stereo out. Therefore it is not useful to feed the right input, unless you have a stereo signal up flow.