GL5 is fine. It's speced for hypoid gears, extreme pressure.
The best thing about the 200ES, along with the shaft drive and reverse, is the low range. I used low range all the time, especially around the house while pulling a trailer. It isn't just about being able to pull more, it makes it that much easier on the trike and when trail riding, I can take off in second if I want.
I know the 250ES has more power, but it's also 100 pounds heavier and doesn't have a low range. When on steep or rocky trails, the 200ES low range really shines.