For safety and legal reasons you have to be "competent" to work on a gas supply or appliance. Unfortunately, the law does not define "competent" as far as a non-professional (i.e. somebody not being paid for the work). If you haven't done plumbing work to a good standard already, then it is not really a wise idea to attempt the disconnection yourself. If you were to get it wrong, use the wrong materials etc. you could end up with a gas leak which may not be immediately apparent but may appear later. It's one thing to read up on this, but another to actually do the work.
If you look at threads on here for the last few days there are people with a lot of practical experience who are unable to solder plumbing fittings. If that were happen with gas, the results could be lethal, not only to yourself but others.
I am not trying to put you down at all, but since this is your first house, it is not the best DIY job to begin with.
What you can do is to get a CORGI registered fitter in to disconnect and cap off the gas supply. You could then remove the appliance yourself, since it's being thrown out anyway, and make good. That would still save you a bunch of money, since the removal and making good will take a lot longer than disconnecting, and of course you pay for time.
.andy
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