After fuel fill-up strong gas smell when car left in garage--what is cause.?

Sounds like you have a seal at the top of tank area that is leaking, may be the fuel sender/fuel pump seal, has worn out and cracked, when fill and drive the gas sloshes and leaks out and causes the smell till enough fuel is used to lower the level where slosh is minimal. Take to shop for repair ASAP. Fuel leak is nothing to wait on.