I would say that anything beyond connecting an appliance to existing fitted gas lines should be explicitly off topic in the FAQ purely for safety reasons.
Definitely not. Banning questions doesn't make anyone safer. It's much better to let them ask so that they can be told if something may be unsafe. Think about this: if someone is searching for info on doing something dangerous, would you rather they found a similar question with an answer clearly spelling out the danger, or would you rather they found no info at all?
The way to handle these is to answer the question if you can, with whatever warnings and caveats you feel appropriate. If someone else posts something you feel may be unsafe, go ahead and say so in a comment.
I'm not sure whether I'd go all the way of calling it off topic, but the answer here is likely "get professional help." Part of DIY is knowing when you can't do it yourself. Perhaps a generic question of "when/where can I work on gas lines without hiring a professional" could be asked, and redirect everyone asking a similar question there would be appropriate.
In the US, most homeowners are allowed to make repairs to what would normally need a licensed trade. The proviso is that they pull a permit and get an inspection.
I think we should remind them that the advice is subject to review by the local code inspector and that the permitting process is for their benefit.
Echo BMitch "DIY is in the eye of the beholder.." Some people can't safely use a hammer and a nail. (I prefer pneumatic, myself)
In the UK you have to be a member of “Gas Safe” to work on gas piping. However as a DIYer it is reasonable for me to wish to know if my current gas pipe is large enough for a new boiler, before I spend the time getting detailed quotes.
(E.g. when designing a new kitchen, the type of boiler that can be put in at a later stage may effect how I lay out the kitchen.)