Embedded in the midi track(s) is volume at the start of the song.
I can see this in Apple Logic (green vertical line at the playhead). (The Step editor or Event editor will show this information).
And it looks like it is possible to export a midi track, so you could delete the volume setting and export it.
I am not familiar with GB to know if you can do this process or not, but if it is not possible (should you wish to do this), another DAW or midi editor app would probably do the same.
I hope this information is helpful.
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