Actually, at the battle of Kursk there were a number of D models that were equipped with the 37mm gun packs; the only official G model was in the hands of Rudel. Also, the G model was armored similarly to the IL-2; and while both had 37mm cannons available, the German 37mm had 6 rounds per gun & used tungsten ammunition (we know what the IL-2 has in the game).
So, the D-3 we have could be updated to have 37mm gun packs with little bother if HTC wanted to, it could also be modified to give us the D-7 with 2x20mm cannon if they wanted (good to kill M-3s & 8s with). The G could be introduced without too much hassle I suspect.