Randall Becker just announced a version of git for NonStop. Its official release is to be in January, and the contact for more information is
sa...@nexbridge.com
If you are on LinkedIn, you might be able to view this discussion:
https://www.linkedin.com/pulse/21st-century-scm-git-guardian-randall-becker
That version apparently has some extra features for maintaining a parallel OSS and Guardian version of each branch, or something like that. I don't remember any mention of a GUI for it.
If you can get by without a GUI, the regular version of git is available in the open source library for OSS on NonStop that Connect maintains. That version runs in OSS and is free. Guardian programs can be built from within OSS, so don't dismiss it out of hand if your need is to built Guardian programs. If you want to check it out, you can get to that open source library using this link:
http://ituglib.connect-community.org
There are pre-built binaries available for H and J series systems and for L series systems. If you want to build it from source yourself, you can do that, too, but it requires more work.
There are some other, old version control systems available for NonStop, but I don't know much about them. If one or the other version of git is not suitable for some reason, post again explaining that, and someone probably will be able to tell you about those others.