There are probably many reasons for the gas smell in your cabin. First off, if a car is running optimally, there should be minimal or insignificant ingress of gasoline smell into the cabin. sometimes , a car may for the initial secs of starting the car, you can smell some gasoline out the exhaust, unburned gasoline due to the car engine being cold.
Last year, I had changed my spark plugs for the 5 year tune up service, but I used an incorrect set of plugs, and apparently it matters that the OE mercedes plug be used, and not ones you buy from NGK that "fits" the car. After using not the mercedes OE part, gasoline smell could be detected clearly after starting the engine, and would last for a while even after driving for a while.
After swapping back to new mercedes OE plugs, the gasoline ingress smell went away.
The other things that can cause unburned gasoline , is the condition of the combustion chamber, carbon deposits on oxygen sensors, intake valves, injectors and piston head. these GDI engines are built with such tight tolerances, that slight deviations, leads to inefficient fuel combustion. I typically put in two consecutive bottles of a good fuel system cleaner every 3 months (e.g. Redline SI-1) and I can testify that it really helps. Even with the top-tier gasolines, inevitably carbon deposits will build up (albeit slower with top-tier gas), hence a thorough cleaning with high performance cleaners like Redline, goes a long way, especially if you plan to own the car and keep it for years.