Do the key signatures C-flat minor/G-flat minor exist?
Theoretically, they can be said to exist, but since these keys are the relative minor keys of E♭♭ major and B♭♭ major, respectively, the key signatures involve double flats. They are impractical and weird. Furthermore, they are (on any 12-tone keyboard) enharmonically equivalent to B minor and F♯ minor, respectively, so there's really no point in using them.
If yes, what notes are used in them?
C♭ natural minor: C♭, D♭, E♭♭, F♭, G♭, A♭♭, B♭♭
C♭ harmonic minor: C♭, D♭, E♭♭, F♭, G♭, A♭♭, B♭
C♭ melodic minor (ascending): C♭, D♭, E♭♭, F♭, G♭, A♭, B♭
G♭ natural minor: G♭, A♭, B♭♭, C♭, D♭, E♭♭, F♭
G♭ harmonic minor: G♭, A♭, B♭♭, C♭, D♭, E♭♭, F
G♭ melodic minor (ascending): G♭, A♭, B♭♭, C♭, D♭, E♭, F
The descending melodic minor scales are the same as the natural minor.
Are they rarely or frequently used in classical music?
They are certainly extremely rare in classical music. In fact, I would be very surprised if they have ever been used at all.