OK here goes;
The G20 was a front induction engine that came in a number of capacities – 15, 19 and 23 I think. Thus a G20/15 is 2.5cc. There is currently on eBay, a very early G20/15, with a separate bearing housing (similar to K&B and ETA racing engines), but the most common G20 has a one piece crankcase.
The G21 was a rear induction racing engine that also came in at least 2 capacities – 29 and 40. The G21/29 is 5cc.