#1
ok so i know all the modes of Gmaj
so if i wanted to play in Amaj, could i just play all the same scales 2 frets higher?

thanks
-|-\/\/()/\/_V/\/\()|_|(
=============)
#2
Yes. That is exactly how it works.

But that's not modes. It's just the different major scale patterns over the fretboard.
Si
#3
Indeed, you just move the scale shapes up 2 frets.

But be advised, that you're not playing in 7 different "modes" in G Major, but you're just using 7 different shapes all over the fretboard that are all in G Major (or the G Ionian mode). Playing different modes of the G Major scale is something else, something more complex.
#4
If you're playing the A Dorian scale shape over G Major, you're still playing in G Major. If you play the chords of G Major in a way that they resolve to the A Dorian (i.e. Am - D7 vamp), then you're playing in A Dorian.