No rule for this one
No ending or word group predicts the article of Gürtel. Learn it as a pair and say it out loud: der Gürtel.
der Gürtel
It's der Gürtel. Gürtel is masculine, plural die Gürtel.
Der Gürtel ist schwarz.
The belt is black.
No ending or word group predicts the article of Gürtel. Learn it as a pair and say it out loud: der Gürtel.