I have a Priority 600 bike with a pinion c.12 gearbox and a Gates belt drive, and I'm currently contemplating winter training plans. Will this work with a Tacx NEO 2T trainer, provided that I buy a gates freewheel cog to fit where the cassette is supposed to go?
No, sadly that hub won't be compatible with the trainer. Your trainer expects a cassette of 9-11 or 12 speeds, which presumes a chain.
Full specs are at https://www.garmin.com/en-US/p/701670#specs
That hub you listed clearly has a thread on the inside, so is for a freewheel. In the text it says "for thread-on freewheel hub"
I believe you're going to need a rear cog for your type of drive belt, that matches the splines on your trainer's freehub.
https://www.gatescarbondrive.com/products/rear-sprockets?product_line=CDX could be a good place to start.
Your other option is to get a cheap chain-based bike and permanently mount that on your trainer.
In order to make this work, you will need a HG (shimano) spline Gates sprocket of the correct type for your belt, a singlespeed conversion sprocket kit with lots of thin spacers and also be confident adjusting the beltline properly to match the Pinion (using the spacer kit).
You will also need to be confident adjusting the belt tension as I have found it changes between different wheels, possibly due to axle dimensions or other component tolerances.
My top tip is to install a normal chain sprocket on the trainer hub so that you can use a normal chainwhip to undo the lockring in the future. Locate it so that it won't interfere with your belt. If you don't do this you will need to have the gates whip or make your own with an old belt whn you want/need to disassemble.
I have been advised separately that I need 2 things: a new sprocket and cassette spacers.
I am going to talk with my local bike shop about these as well. If I try this out and get this to work successfully, I'll mark it down here for future reference.