Hybrid Water Heater with Gas Backup?
The problem I see is that a gas water heater has a big pipe/flue running through the center of it which takes away a lot of room that the hybrid system probably needs. It also vents very hot gas out of the top which is where the compressor is mounted in a hybrid. The electric backup elements are tiny in comparison and just need to stick into the side of the heater.  People with gas, which is normally cheaper than electric, don't have as compelling of a reason to choose a hybrid heater, so I doubt there is much demand.

So, very complex and low demand...  Probably doesn't exist.  I've never seen one.