When starting up my car has dark smoke, not black but it's gray and smells like

What is your question?

Just bought a BmW from a dealership, the car is smoking at startup. Only had for a week and just started when I parked it today and went into the post office. It's not white smoke, and has no smell at first. When driving home it went away, then we waited like 30min to start it up. After starting it up it smoked for a lol bit and smelt like gas. What could be the problem