It's one skill to know that the minor sixth above Bb is Gb. There are many ways one might get to this. If you already know your major and minor scales, these answers should be fairly automatic. If you trouble with sixths and sevenths you can use the inversions (minor sixth above = major third below etc). If you know your circle of fifths really well you could use the perfect fifth as a reference point.
It's another skill to know what that Gb should sound like. And that is where relative pitch, aural training, etc. come in.