Depending on how long that horisontal pipe after the trap runs before it turns down you may have a buildup of grease/food/stuff that needs to be pulled out. It looks like you have pretty good access to the pipe connections/unions, so take it a part, make a tool to reach into the pipe and pull out the stuff. I used a piece of stiff wire and made a hook on the end of it. Push it in the pipe and pull it back out and see what you find.
This same same happened to our sink. Tried everything to get the smell out but in the end I took it all a part and cleaned out. You wouldn't believe the crap I pulled out. HAZMAT should have been called.
