Driving without a gas cap is not dangerous but if you drive without a gas cap debris and water from the outside can get into your gas tank and then clogged your fuel pump, fuel filter and fuel injectors.
If you have lost your gas cap you should go to Oreilly or other auto parts stores and buy a new gas cap for your vehicle to prevent water and other debris from getting into your gas tank.
There's also a valve in newer cars to prevent the fumes and gasoline vapors from escaping from the gas tank and your engine will have no harm done to it nor will your vehicle have any harm done to it without a gas cap.
But I would recommend getting a gas cap and if you get a gas cap get a locking gas cap to prevent theft and also if you don't have a gas cap at all on your vehicle it makes it very easy for someone to just stick a hose into the gas tank and siphon the gas out.