You need to go about 10% higher with gasoline as a gasoline engine of the same displacement does not produce as much power as a nitro/alcohol engine. So for a 1.20 glow engine the equivalent gasoline engine would be 1.3 cubic inches. However, most gasoline engines are measured in cubic centimeters, not cubic inches. SO the conversion is:
1 cubic inch = 16.39 cc
So 1.2 glow == 1.3 gas = 1.3 cubic inches = 21.3 cc