I could see it helping out with rythym/timing, but that's about it.
It really doesn't scratch the surface of dexterity/muscle memorization required in forming chords, changing between chords, recognizing notes/chords by sound, musical theory, ect.
With GH you have 5 buttons at the top of the neck that you hit. That's it. With a real guitar you are looking at 6 strings and ~22 possible positions(frets) to play those strings. And then playing any number of those strings at a time in virtually infinite number of positions. And to further confuse you have different techniques you apply to get different sounds that you really can't do with the GH controller (hammers, pull on/off, palm mute, ect). You just can't compare really.
Same thing with Wii sports. Does bowling on a game console with a remote that I swing make me a better bowler in real life? No freaking way.