The former Griquas and Cheethas flanker has targeted Aironi's final game of the season against the Ospreys as his last game.
"Gareth is a great player and a great guy," said Aironi's director of rugby Franco Tonni.
"He is probably one of the best foreigner players arrived to Italy in the last five years.
"The door will stay open for Gareth: if rules change or if Aironi will need help during the season, he could be back in Viadana. Meanwhile, we just can thank him for all his work on and off the pitch."