Sounds like a BS answer from the gas conversion fitter to me, the auto gearbox will always load up and slow down the engine when drive or reverse is selected and if the system were still on petrol the engine management system will compensate by trying to increase the idle speed to actually keep it the same speed. The gas system obviously does not have this compensation, you should go back to them and state that the autogear box was perfect when on petrol (it was I suppose??) and that the system they have installed is not operating correctly and that you want it corrected.