Did you find the literature from Gates about the belts that was in with the kit of parts?
Usually there are two ways to tension a belt: you make it so the shaft that supports one of the pulleys can be moved away from the other pulley, or you add a third pulley called an "idler" that just pushes the belt from the side and keeps it tight.
The only toothed belt I have handy for reference is this one on my car
it uses an idler that pushes against the inside of the belt.
