It would probably be possible to turn off gas to areas or streets.
I'm sure they have plenty of provision for safe isolation. The problem is that once you cut the gas supply, you can't turn it back on again until you've at least closed the shut-off valve at each premises, otherwise you risk gas escaping from some appliance that isn't clever enough to turn itself off in the absence of combustion, and potentially going
BOOM.
Regardless of whether you take the nice-nicey approach of waiting for everyone to let you in, or using police-assisted gasman powers of entry, that's the sort of thing that takes ages, is expensive to do and pisses lots of people off.
Sort of thing that might actually be a reasonable use of smart meters. If everyone had smart meters.