Why did the check engine soon message light come on after I got gas?

If the gas cap is not installed correctly and or sealed, you create an air leak and the system will detect a fault. If you know the cap was not installed correctly, then the simple fix is to reinstall properly and after a few drive cycles the light and or fault on the instrument cluster will go out. If the cap was installed correctly, then the cap seal might be worn, dry and cracked and you would need to replace the cap. If that doesn't work, then there is another fault code causing the CEL to be on. Scan the car for the codes with an OBDii scanner, or an auto parts store will do it for free. The code will then give you an indication on what the issue is. Good Luck.

Buy an OEM gas cap if that's the case , spend the extra $$ so you know it's not crap . The clerk at auto zoo is not a technician , they sell parts .