There are many other factors at play when going from Glow to Gas...

I've found that to obtain similiar performance as a 1.00 - 1.20 glow engine I need to go up to a 26cc gas engine.

The Gas engine is heavier and requires an ignition pack. This adds to the plane's weight and increases the power requirements.

On a power per displacement basis, glow does better than gas, but gas is much cheap to run... so you normally have to increase the displacement to get power equivalence when moving to gas.

You also must adjust prop sizes and pitches accordingly too.