Recently talking to an engineer, a natural gas line welder, and some guys on a gas leak response crew from our utility company.. regarding my earthquake preparations. I'd gone to great pains to get a non-sparking gas meter valve shut-off wrench, and was proud of myself. These guys, unanimously, suggested that I add to my kit the following:
Two or three 3/4" 3' wooden dowels, sharpened on one end. Two or three old T-shirts.

When encountering a gas leak in a ruptured pipe NOT readily controlled by a valve they suggested using the T-shirt wrapped dowel crammed into the rupture, to stop the leak temporarily.
I though they were jacking with me, but they all agreed that in fact "that's actually what we use" .. perhaps with some duct tape overwrap.. until they can safely excavate back to the nearest working valve.

Never would have thought of this.

"We have the technology"

You of course realize that most small (say 3/4" and down) gas lines are at VERY low pressure - you can often cap them by putting your hand over the opening, while someone gets gear?

and yeah, they usually have wood plugs of various sizes - just remember to bang them in with either another block of wood, or a non sparking hammer

I wonder if Cork would work, or if it's too porous?
They stop up bottles with fluids that tend to build up gas pressure, so I guess it would work.

My thoughts were that cork seems like it would be easy enough to jam in a leaking pipe quickly if it were big enough, it's light, and I have a whole drawer full of culinary knives with their points stuck in wine corks of various sizes. LOL