Someone once told me, "Gates belts shrink and get tighter when they heat up." This is, at worst, wrong and at best, a misunderstanding of what really is going on.
Gates has a recommended tension for all their belts. For the belts typically used in aircraft, a rough estimate is 1/4" play per foot of unsupported length. This seems loose, for those used to V-belts. The problem is that typical aircraft cogs are so large that when they (not the belt) heat up, the coefficient of expansion due to heat is enough to over-tension a belt that was already tight when cold.
Gates belts don't shrink with heat, but large metal cogs expand. It is best to set the belt to the tension after a significant warm-up.
Yes, perhaps I should have put that at the top of my post rather than at the end. To put my own experience in practical terms, I often took off with a "relatively" loose belt and returned with one that was "banjo tight."