By calling a licensed master plumber / licensed gas fitter / Gas supplier
You can stop a leaking gas tank, by making a paste of soap on the hole of the gas tank.
If it is leaking when you use the shower, not shutting off completely, it will need to be replaced. If it is leaking at the wall, take it off and reseal the thread on the pipe with either pipe dope or Teflon tape and reinstall.
If it's a green corroded pipe then it's old copper. -You can't fix it. Replace all the length of corroded pipe with PEX and a union at each end
More information needed. Is it visible, is it underground, is is plastic, is it metal pipe - -
Take it to a trained technician
take it to a mechanic, a gas leak is nothing you want to mess with.
Pressure testing with an inert gas after the line is isolated from the supply
Demo a section of the wall.
If your vehicle is leaking fumes, then your gas tank needs to be fixed.
You must break or cut the wall at that point. No other option
Bad fill pipe?
chop it up with an axe then use duct-tape