When you run the fridge on gas make sure the 240v is turned off. These fridges take a long time to start. Within 2 hours you should be able to feel the pipework on the back heating up and by three hours you should have some cooling inside although you should allow up to eight hours before it reaches its lowest temperature.
If it doesn't happen like this you may have a partial blockage in the gas delivery system which would need to then be serviced.
Cliff
November 2008