Can you post the model number please.
Also has the fridge started to loose it cooling?
Is the smell oilly?
Ok as you say the cooling is fine then i dont thing a gas escape will the issue here. As if it was then you will loose the cooling inside the fridge as its the gas being pumped around the system that does the cooling in there.
So see how it goes but if the cooling is fine inside there then it wont be the gas thats the smell in there.
But what can happen on these built in appliances is the condenser fan can fail under there and this causes the compressor and condenser to get too hot and can give off a bit of a smell.
Or you can get a build up of muck in the drip tray at the back near the compressor and this gives off the smell as well.
What you can try if you are able to is pull the appliance out from under the unit and give it a clear round the back. Check the drip tray and give this a clean as well.
Also while its out check the condenser fan and see if its running. Now you can check this without pulling it out as listen very carefully at the bottom and see if you can hear it running as well as the compressor.
Try that and see what happens but from what you say i dont think the smell you have is gas as if it was then the fridge wont be cold at all so you can rule the gas leak out on this.
No worries thats fine. And yes you can report back any time once you have checked it as thats not a problem.