To play in G off the D row you'd do what Kitty Hayes did to play in F off the C row but it might need a C natural as an accidental... on the index finger of the accidental row of a 21 button to get along with most other players in G.
Well you have various options. Even a 20 -key D/A could have a non-standard layout allowing the presence of a C natural. But if no C natural, you can get around G tunes in the several ways that 1-row D melodeon players do :-)
D/A concertinas with more than 20 keys will almost always have one or more C naturals available. IMO a very handy place for that C natural on a D/A instrument (though not one of the standard traditional anglo layouts) is an *extra* button at the top of the LH inside A row (above the button for e/f#) played with the LH index finger and allowing C natural in both bellows directions. This can sometimes be added to a 20 key instrument -- the Dippers rebuilt a Jones that way for me.