You picked a good example because by key signature circle-of-fifths distance from C major, the choices are equidistant. Gb-Bb-Db can be obtained in the key of

You picked a good example because by key signature circle-of-fifths distance from C major, the choices are equidistant. Gb-Bb-Db can be obtained in the key of Db: Bb, Eb, Ab, Db, Gb. It is that key's subdominant. F#-A#-C# is in the key of B: F#, C#, G#, D#, A#, and is that key's dominant. Five sharps or five flats. Same distance from C major.