In RC hobby fields, RC gas engines are quite different from glow engines. A basic difference is the application. Gas engines are more suitable for large scale aircraft models while glow powered engines are much proper for small sized models. Not to mention other features like structure, cost and operating system etc.

Usually if a plane weight over 5kg or even larger, to choose a gasoline engine is more cost-effective as it consumes less fuel. Generally speaking, the differences between these two kinds of engines can be shown as following:

Ignition Systems
Spark plugs are used for gas engines to ignite the fuel while glow engines used glow plugs. Most RC gas engines use the electronic ignition system. They can provide electricity to the spark plug at correct time to maximizing the power of engines. And a magneto ignition system is generally hard to start than electronic one.

Fuel using
To produce same amount of power, it may need a little larger gas engine, which means gas engine takes more weight than glow engine when producing the same amount of power. The reason is the methanol/air mixture produces more energy than gasoline/air mixture during combustion. But glow engine is more fuel-cost than gas engine that's why most giant scale planes are gas powered engines.

Radio Interference
Ignition systems of gas engine may cause radio interference thus the ignition systems had better be away from the radio receive for about 12inches. That's an evident difference from glow engines.

Generally speaking, gas engines are easier to start and fuel-saving. They have built-in fuel pumps which perform more reliable. Glow engines are fuel-consumption, and cannot be used for large scale models. That's to say, RC gas engines have a lower power to weight ratio, need ignition system to power the spark plug, and are generally more difficult to operate compared with glow engines. But the cost of gas engine fuel is much cheaper. People with RC hobbies may get used to gas engines as the added complexity and weight of RC gas engines is useful for more affordable fuel when flying a larger heavier airplane.