As Rcpilet said, the glow version (calling it gas will confuse people who use glow and gas to differentiate between the two main types of fuel used in airplanes) will be built heavier so it can handle the extra vibration of the engine. It will also have room for a fuel tank. The electric one should be lighter and have a place for the battery.

Choosing a kit such as a P-51 for your first build may not be such a good idea. Also, while I have ordered lots of stuff from Tower, they are not the best place for ordering kits. They stock what sells well, and these days ARFs and RTFs are what seems to move. You'll probably have to do as already suggested, spend some time looking around in the electric forums to get some good ideas. I know the electric crowd has done quite a few conversions of planes in the .049 sizes to electric. There are also lots of kits out there that you will never find by looking at the major retailers such as Tower, Hobby People, etc.