The G Major scale is normally the second major scale I teach students and follows on very well after learning the A minor scale which is very similar in shape (having one sharpened note at the top). The G Major scale has the same fingering as the A minor and C major scale and I would suggest starting with 1 and 2 octave hands separately followed by hands together once this has been mastered. You can increase the G major scale to 3 and 4 octaves by putting the thumb under again in the right hand and 4th in the left hand and repeating the fingering pattern again. In the earlier ABRSM piano grades you will only need to play the G Major scale 1 and 2 octaves hand together however in later grades you’ll be expected to play it up to 4 octaves hands together. Although a lot of lesson books cover scales, it is well worth buying a seperate scale book for the ABRSM piano grades to refer to if needed. ABRSM piano scale books can be bought online as well as purchased locally from music shops in Swindon. 

Here is the piano fingering for the G Major scale as taught in my lessons:

One octave R.H (up) 12312345 (down) 4321321

One octave L.H (up) 54321321 (down) 2312345

Two octaves R.H (up) 123123412312345 (down) 43213214321321

Two octaves L.H (up) 543213214321321 (down) 23123412312345

Watch video demonstration of the teacher playing the G major scale: