They both run on different types of fuel....A gas engine is like a chainsaw engine...Runs on normal pump gas that you get for your car mixed with oil.
A glow engine runs on glow fuel which is usually a mixture of nitromethane and castor oil or synthetic for most sport fuels.

If you're in a conversation about electric vs. gas you could assume it means glow and gas fuels.
But if you're comparing glow vs. gas....Technically they are both "gas" but they use different fuels and have different purposes.

Gas engines are also 4 stroke ( they do make 2 stroke but I have never seen one on an airplane)
glow engines have 2 stroke and 4 stroke types