After the conversion fails ~/.config/ChiTuBox/tmp.gf.tmp and~/.config/ChiTuBox/tmp.gf will be gone, but you will be left with the ~/.config/ChiTuBox/tmp.gf.link file. You can then manually run the Longer plugin like this:
Thanks for the post! Helped me work around the issue. I came up with (I think) a simpler method using your advice though. When you go to save the file, instead of .lgs/.lgs30 simply change it to .gf and export it that way. Then you can manually run the converter as you showed.