The enharmonic of Gb is:
O A. F#
O B. G
O C. F
O D. G#

It could be either F# or G#

Explanation:

If you’re familiar with the circle of fifths — or you just know your way around the key signatures — you may have noticed a few anomalies. For example, keys like B-sharp and F-flat major are seemingly absent, while others go by two names. When comparing C-sharp major and D-flat major, it's easy to see these notes are exactly the same.  

C-sharp major: C#   D#    E#    F#   G#   A#   B#

D-flat major:     Db   Eb     F     Gb   Ab   Bb    C

Likewise, their respective relative minors are also identical in tone.

A-sharp minor: A#    B#   C#    D#    E#    F#    G#

B-flat minor:     Bb    C     Db    Eb     F     Gb    Ab