Is it safe to use “PushFit” fitting on gas pipes?
In the UK, it's not legal (quite correctly, in my view) for householders to work on gas pipework or fittings at all. 

If a buildings inspector or surveyor spots pushfit fittings on gas you can expect plenty of trouble down the line... to say nothing of the risk of a pinhole leak or if there's another fire nearby that melts the fitting.

While you might save a few pounds in the short term, do you want to risk your life and that of those around you? A leaky joint in a water pipe might wreck a ceiling or a floor, the same in a gas pipe might kill you. 

Head here for details of the Gas Safety scheme which holds a register of tradespeople who can do the work for you.