What packaging? Plain .DEB? Flatpak? Snap? AppImage?
Otherwise, the bookmarks are stored in ~/.config/gtk3/bookmarks (even for GTK2 apps such as Gimp, it seems...). You can edit the file and add a bookmark manually if you want to investigate (the bookmark will be shown only of the directory exists).
If you are using an "alternate packaging", these things are often sandboxed (especially flatpak/snap) and see a virtual filesystem which may not be what your shell file explorer see, so the bookmark file could be elsewhere. I has some success a long time ago by establishing a link (or what is a bind-mount) from a regular directory into the filesystem seen by the flatpak version. This allowed be to share the recently_used.xbel file between Gimp and other apps.
Otherwise, the bookmarks are stored in ~/.config/gtk3/bookmarks (even for GTK2 apps such as Gimp, it seems...). You can edit the file and add a bookmark manually if you want to investigate (the bookmark will be shown only of the directory exists).
If you are using an "alternate packaging", these things are often sandboxed (especially flatpak/snap) and see a virtual filesystem which may not be what your shell file explorer see, so the bookmark file could be elsewhere. I has some success a long time ago by establishing a link (or what is a bind-mount) from a regular directory into the filesystem seen by the flatpak version. This allowed be to share the recently_used.xbel file between Gimp and other apps.