I have a couple of thoughts. First, The gas Gator 620i has nowhere near the power of the 850d diesel, so we really cannot compare those two with regard to hillside performance. Second, the engine will take 30-50 hours to become broken in, so you might get a little better performance after some use. Third, there is a chance the brake might be hung (I had one that had this problem) and occasionally the parking brake sensor will not activate. The other possibility is apparently there is a spring mechanism within the CVT belt tensioner that can occasionally loosen, resulting in poor performance when under a load. Both of these common and simple things could worsen your hillside performance in a fashion such as you mention. I would DEFINITELY take it to the dealer and get them working on this to troubleshoot. Do not wait for the break-in time because while that will help some, it will not make a dramatic difference.
The final, and sadly, non-repairable reality of the 620i is that it just has no power. 23 hp in a machine that pushes 1400 pounds field ready with a CVT tuned to a 30-32 mph top speed equals a machine with lackluster performance. I have had three different gas Gators, two HPX models and one XUV model, with the Kawasaki gas engine. I got them at good prices and promptly traded or sold them for the diesel, which has much more power (read, torque). I live in a hilly area and the gas Gators are really weak when one puts them on steep hills. The problem with the CVT also is that to keep the engine within its fairly narrow powerband it will continue to slow the groundspeed of the machine, and sometimes slows it to almost walking speed to help the underpowered engine keep up its RPM's. This is not noticeable on flatter ground or shorter hills where uncle Mo can be used, but on the long or unusually steep grades it is a real drag. The short message here is that there are some simple checks and fixes that can be done, but none of them will likely make this machine a star performer. They are durable and very nice, but if you want the power go diesel--until summer of 2010 when the new model comes out which will have double the horsepower--or you could get a Ranger. I have both a diesel Gator and a Ranger and both of them will show some serious back end to a gas Gator.
John M