Answer by Albrecht Hügli for What is the symbol for "is the enharmonic equivalent of"?

F# - Gb or F#/Gb

(I know you use the slash to indicate a secondary dominant or any other degree of another degree).

https://www.theorie-musik.de/grundlagen/enharmonische-verwechslung/

Edit:

I agree that / is used to assign F/G = F chord above G. So how about back slash: F#\Gb

After reading the other answers here I think a good and clear solution will be euqal in parentheses: F# (=Gb)