No rule for this one
No ending or word group predicts the article of Grad. Learn it as a pair and say it out loud: das Grad.
das Grad
It's das Grad. Grad is neuter, plural die Grad.
Es sind zwanzig Grad.
It is twenty degrees.
No ending or word group predicts the article of Grad. Learn it as a pair and say it out loud: das Grad.