Riaz is a municipality in the district of Gruyère in the canton of Fribourg in Switzerland. Riaz is first mentioned in 900 as Villa Roda. The municiā¦Riaz is a municipality in the district of Gruyère in the canton of Fribourg in Switzerland. Riaz is first mentioned in 900 as Villa Roda. The municipality was formerly known by its German name Zum Rad, however, that name is no longer used.