No rule for this one
No ending or word group predicts the article of Glas. Learn it as a pair and say it out loud: das Glas.
das Glas
It's das Glas. Glas is neuter, plural die Gläser.
Ein Glas Wasser, bitte.
A glass of water, please.
No ending or word group predicts the article of Glas. Learn it as a pair and say it out loud: das Glas.