Despite not contemplated in the official docs, I found that cloning the repo and running e.g.
works fine thanks to pyproject.toml.
The GSAS-II GUI startup script generated in $VIRTUAL_ENV/bin by
[project.gui-scripts]
GSAS-II = 'GSASII.GSASII:main'
does not find the right file however; changing it to
[project.gui-scripts]
GSAS-II = 'GSASII.GSASIIGUI:main'
does the job. Is this a bug or am I missing something? Thanks!