What is the efficiency of running an electric motor from a gas generator?
I know a lot of small aircraft engines weigh around 150kg and several
  use gear boxes as well.


Almost no piston aircraft use gearboxes. The only one that I know of that ever did was the Porsche Powered Mooney PFM which was largely a failure overall, with the gearbox being often cited as an issue. Smaller turbo-prop planes do use gearboxes due to the super high RPM range of a turbine but those are generally larger than the ones that seem to be in question here. 


  So Does this mean you could save the weight to make the plane electric
  but not battery powered or is the generator simply going to be too
  inefficient?


As mentioned the issue here is simply weight and line loss. You are taking mechanical energy converting it to electrical, bussing it somewhere then converting it back to mechanical. There will always be line loss in a process like that when you don't really need it (since you need to spin the prop to begin with). Gas-Electric (be it diesel or gasoline) is advantageous as you can drive full torque at 0-RPM which is useful when getting a giant freight train rolling but does not provide a ton of benefit to light aircraft. You are also introducing at least 3 major points of failure (engine -> generator -> motor) to a system that previously only had one.