I don't know what the recommendation is for Gamma engines but my experience with other engines (e.g. Fiat/Lancia/Alfa Twin Cams and Maserati V6) is that you never re-tension a belt once fitted. In fact you are meant to replace the belt if the tension is ever released after the belt has been in service. But the instruction for fitting belts is that you tension the the belt, manually turn the engine 3 revolutions and then check the tension again and adjust if necessary. I find this manual turning of the engine tends to take up any slack.
Mind you, I haven't checked my Gamma's belts several hundred miles after fitting so I think I better do that...