From what I understand about these two speeds, they are both control related. They are based on the critical engine failing.
VMCG: The minimum control speed on the ground during the take off run.
VMCA: The minimum control speed in the air that an aircraft is controllable.
Both of these speeds decreas with increasing airfeild elevation and Pressure Altitude. Reduced take off thrust and a large fin will also reduce VMCG.
I cant see any relation to weight, but it would be interesting to know any different...
