G major is a very easy key to play on a melodeon which has a G row! All that is required is learning the push/pull pattern which gives the notes in the right order.
The scale will start on button 3 of the G row.
To play the scale you must play:
Button 3 on the push (G3)
Button 3 on the pull (G3')
Button 4 on the push (G4)
Button 4 on the pull (G4')
Button 5 the push (G5)
Button 5 on the pull (G5')
Button 6 on the pull (G6')
Button 6 on the push (G6)
Notice that the push/pull pattern reverses for the last two buttons!
Once you can do this pattern ascending, try it in reverse.