It depends where the composition modulates from the home key. If Schubert had written the third of his Opus 90 impromptus in F-sharp major, by the fourth measure we’d already be looking at G-double-sharp, with many more cumbersome accidentals to follow. In G-flat, that G-double-sharp is simply an A-natural, making it much easier for the composer to write and the pianist to read. There are three major keys that have this “dual nature":  C-sharp/D-flat, F-sharp/G-flat, and B/C-flat.

Here is Schubert’s exquisite Opus 90 Impromptu in G-flat major.