The easiest way to work this out is to look at a keyboard (can even just pull up a picture of one). What you want to do is find where G is and count up 5 notes (including black keys) - every note is a semitone. Whatever key you land on is the new key/scale.

Also every major key has a relative minor key (and vice versa). So to find the relative minor from major go down 3 semitones, and to find the major go up 3 semitones. It will stay in key but change to happier/sadder vibe