Much like a normal chain, there isn't one correct answer. So long as the belt stays in place and does not slip, there is a wide range of acceptable tension. With too little tension, the belt will skip; too much tension creates noise and is harder on bearings.
The best tension will depend on the specific model it is fitted to and also rider weight and riding style. Note: Aurora Limited models will require a lower tension than most other models.
You can download an app for smartphones to tension it according to sound. You can download the app there, and if you use it, you want to aim for a value between 35-50 Hz (usually lower is better).
You can also tension by using a deflection test. Normally this is done by hanging a 10-pound weight from the middle of the belt and adjusting to 1/4" to 3/4" deflection (depends on rider weight, model, and riding style). You can makeshift this by pushing down on the middle of the belt with one finger and seeing what the deflection measures to.
Gates also makes a belt tension tool called the Gates Krikit Tool. Those can be ordered/purchased at many bicycle shops or online.
Here is a video about belt tensioning from Gates belt: