Natural gas smell in house after new roof installation
If in ANY doubt about whether there is a significant gas leak, evacuate, alert the fire department, and tell the gas company to send someone over with a sniffer to check for gas and find the source if a leak does exist.

However, remember that there might just be a skunk in the area...

If you're using gas, it's probably worth investing in a gas alarm before this issue arises. Some serve as carbon monoxide alarms too.

(I heat with gas, have cooked on a gas range (the only option I like more is inductive, and that's a bit of a mixed bag), and will probably switch to a gas dryer when I upgrade the laundry, so I'm certainly not particularly afraid of it. But like electricity it should be treated with respect, and if it seems to be appearing someplace it shouldn't, you want someone who knows what they are doing to investigate.)