The tempo in GarageBand is expressed in beats per minute and is set to 120 by default. But you can change it to match the tempo you need for your song. On iOS, you can change the tempo for your song to a specific value. And on Mac, you also adjust it for the entire song or add tempo points to the Tempo Track so that the tempo changes throughout the song.
This tutorial shows you how to change the tempo for a song in GarageBand.