How to center Gates belt drive? The Gates belt is running slightly off-center on the rear sprocket. Does anybody know how to adjust the rear sprocket/wheel such that the belt runs centered?

Thanks
The short answer is that they had a bunch of problems with this on the center track-less version(s), and added the center track to finally solve it.

For the belt to not get off-center, the gears need to be in exactly the same plane.

Rear triangle alignment, dropout alignment, and BB shell alignment are all potential factors. The former two are things you could do something about on a steel frame, but the last isn't. (Shell alignment is also not a common issue, just a possible one.)

Imperfect chainline (or whatever you want to call it when it's a belt, not a chain) also causes it. If possible I recommend you get the frame aligned before even starting in with addressing chainline in this case. The reason is that rear triangle and to a lesser extent dropout misalignment affects your chainline measurements, but chainline can't really be used to compensate for alignment problems here.

The belt also needs to be properly tensioned. But if there are problems with alignment and chainline, tension won't solve them.