I am totally unfamiliar with your newer model Gator, but with all the Deere gas powered machines I have had, both tractor and utility vehicle, the surging at idle is most commonly caused be some debris or dirt in the low speed jets in the carburetor. If you run ethanol mixture fuels you might also have some 'jelly-like' contamination where the alcohol has attracted enough moisture to form an emulsion which also clogs up the fine holes in the idle jets. The other main source or surging is an air leak into the manifold below the throttle butterfly. These root causes may not apply at all for your 625i with its much more modern power plant.
Does your surging/hunting engine speed variation cease when you use higher throttle settings and/or add a bit of choke? Does it even have a choke? You might start by cleaning the fuel system passages and look for air leaks into the manifold -- as you said you already changed the fuel filter...I don't know how much having fuel injection might change what is needed in performing trouble shooting for your symptom on this machine.
Maybe a member here with your model Gator can be more specific. Let us know what you find.
The 625i is a fuel injected gasser. the biggest issue I have seen is a bad throttle position sensor. What does your oil look like? Is it over full? Smells like gas? Remove the spark plugs and have a look see. A dealer with the correct test harness will be needed to correctly diagnose your problem.