Gas actually rots PTFE tape so you cant use that, you have to use a special gas sealant/paste.

I'd be suprised if there wasn't another form of isolation just before or actually inside the house somewhere, have a look for it so you're not removing the pipework you just mentioned with the pipe pressurised with gas.

You should be able to get the fittings you require from B&Q etc, at a guess i'd say it looks something like 1/2" BSP.