Leave immediately
Get everyone (and pets) outside. Do not flip light switches, use the phone indoors, or start a vehicle in an attached garage.
Call from outside
From a neighbour or a cell phone outside the house, call your gas utility 24/7 emergency line. In a major leak or if anyone feels ill, call 911.
Do not return
Let the utility verify the home is safe to re-enter.
After the all-clear
Relight pilots only if you are comfortable; otherwise a licensed gas fitter should handle it.