Nice looking car. That would be a SL320 in 1995 (Mercedes moved the "SL" infront of the motor designation number in 1994 which is also when the six cylinder motor size changed from 3 liters to 3.2 liters... "320").
I'm not very familiar with the six cylinder engines, but there are many members in this forum who will be very helpful. Similar to the other R129 powertrains, the six cylinder powertrains have a few things to watch out for, but if they are well maintained you can enjoy them very much.
Back to the gas smell problem... As always be careful around leaking gasoline and park the car outside if possible until this is resolved. If there is no sign of fuel leaking from the fuel pump at the rear, my next suggestion would be to start looking under the hood. I believe that your car has a very modern electronic fuel injection system with a fuel rail (a metal tube) that attaches to the six electrically controlled fuel injectors. It may be difficult to see all of it with out removing some of the plastic engine covers, but it should be located on the drivers side where the intake manifold attaches to the cylinder head.
There will be two fuel lines attaching to the fuel rail (a pressure line from the fuel pump and a return line to the gas tank). There is also a small fuel pressure regulator device attached to the fuel rail. As some of these components age, they are also known to start leaking. It can be as simple as the o-rings that seal the injectors to the fuel rail or the fuel pressure regulator have failed (not too expensive to have fixed). Or it could be the injectors themselves have lost their internal seal causing the Siemens or Bosch injectors to leak externally at the seam in the injector body and need to be replaced (more expensive to replace... my SL500 had that problem). Edit- If the problem isn't there, the forum will have more suggestions. How many miles are on the car?
Here is a picture of a typical fuel rail with the injectors and fuel pressure regulator attached: