If you draw the circle of fifths, with C major at the "12 o'clock" position, and going clockwise, then the tritone for C (F# or Gb) will be at 6 o'clock. And so on, the tritone of the key at the 1 position will be at the 7, 2 and 8, 3 and 9...
A key six degrees away isn't all that related, but... the C 7 and the Gb 7chords share the two notes, the 3rd and the 7th (reversed), in this case, E and Bb. The distance between this 3rd and seventh is also a tritone. In jazz voicing, the fifth and tonic are often left out, so a chord of these two notes could be ambiguous.
So you can substitute one dominant chord for its tritone equivalent. If you have G7 resolving to C major seven, you can use D flat instead. You get a cool voice leading, with the root of the chord descending a half step, the third descending a half step, (f to e) and the seventh remaining the same (b - b).
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