It's possible to just have Bb and Gb. Look at Bartok's Mikrokosmos for some examples. It may take some wrestling in your notation program (and may not be possible on free ones). However, this is likely to be distracting, and the best way is probably to use a standard key signature corresponding to the closest tonality and use lots of accidentals. What you'...


Why Gb? and not F#? If you have G-Dorian (1 flat) and construct a lead tone F# it will fit.

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