The dc motor will start the gas motor when it is switched on. This allows for easy control of the hybrid system, just switch it on and it provides power. You need a good coupling to do this.
A three phase motor (rectified) is more efficient (I believe) and probably lighter weight for the same power output but will not do the auto start that the dc permanent magnet motor will do. If you have another way to start the gas motor, or just leave the gas motor running all the time, a three phase generator would be preferred. You could also use a belt drive if you did not expect the electric motor/generator to start the gas motor.
Permanent Magnet Alternator Wind Blue High Wind
A gas motor of the typical Briggs and Stratton type will do fine as a generator motor.