Unfortunately there's no real good music editor with the current build. The best that I was capable of was by making the song in beep box (or anything with a midi export). And then using the sample song located in your music folder of gb studio. I loaded the .mod file in open mpt FIRST. Then I went under file and append the .midi file. Then I manually resampled each note, and test that it worked. Then export the new song in the .mod format.
In short, theres just no easy way of doing this (or a good way for that matter) you just have to experiment and see whats right.
Otherwise I used
Beepbox.co for making the music and exporting to .midi
Open mpt for tracker conversions, as to convert to the .mod format