When a singer asks to tune a song a half step down from Gbm would that be Fminor or Gb?
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It means F minor.

Singers frequently change the key of (transpose) a song in order to best fit their vocal range.

Changing to Gb major would be a change of mode, which would entirely change the character of the music.

The below video is timed to a performance of the Beatles's "Here Comes the Sun", first in major, then shifted to minor.
I'd be surprised if the original key was G♭m - F♯m is far more the common way to call it. But you wouldn't change the key of any song from minor to major - or vice versa, just to suit someone's range. In fact, merely dropping a semitone is unusual. if it's that awkward to sing in G♭m, Em could be even better! (And probably easier to play...)

So, basic premise - if the original is in minor, change key to another minor, if major, new key needs to be also major.