Dave, I'm very happy with both that Ventrac tractor and the Kubota engine.
In in dead of winter (Maine) @ -12 F The engine is slow to start. As in any extreme environment it can quickly take a toll on any equipment. It's very hard on the starter motor as well as engine internals. As far as the glow plug "duty cycle" it runs off of the engine temperature. I love the Ventrac snow blower it is the most robust and yet easy to operate machine. The block heater will only make my machine all the more easier to start on the bitter cold Maine mornings.
Where the problem starts goes like this. The block heater warms the engine coolant. The GP controller looks at the coolant temp and says engine is warm so little to no GP time is required.
As the engine starts to turn over it is inhaling outdoor air which may be -30 or -40 and this cold air entering an engine with a warm block will still turn over far more than necessary.
I don't know your Ventrac nor its 902 engine but when you indicate the GP controller is sensing coolant temp you will not have the results you hope for. There are manual GP control on time work arounds not mentioned in the manuals.
Others on this forum have read my advice many times on this topic so sending it to you should you want it in a PM will not bore others.
Dave