Glas is an archaeological site on an island of the Lake Copaïs in Boeotia. Excavations have unearthed a vast Mycenaean citadel and some potteries with inscriptions in linear B.
Athanasios G. Angelopoulos suggests Hyle as the ancient name of the town, relying on Homer in the Iliad: "Oresbios, who wore a brilliant miter, lived Hylè, where he took care of his wealth, in the middle of Lake Képhisside, not far from the rich tribes of the Boi'tians."
The ancient name of Lake Kopais, Kephisis, is accepted by the majority of researchers. And I find that Glas, Gla in Greek, can come from the ancient designation "Hylé".