While the real professionals have chimed in here; I have to add a PSA here as who knows who reads this thread.
If you suspect a gas leak so significant that you want to shut off the gas supply - call 911 and get the heck out of there! Let the pros handle it.
I have seen the damage caused by gas explosions and you don't want to be nearby. While, exceedingly rare given the amount of gas supplied homes, don't take a chance....
CZ...
Damn skippy. I did this a few years ago. Turned out I was downwind of my (idiot) neighbor's leak. Don't recall how they did it. Wasn't taking chances with a fresh baby and co. They went to my mothers at 9pm while I waited outside for Indiana Gas