I'm not exactly sure what "GNU MUMPS" is. Harlan Stenn is the author of PFCS MUMPS, which is a closed source package. Maybe it was open source under that name at one point, but if that's true, I think that must have been a long time ago.

If you are looking at an open source MUMPS, the most popular are GT.M [0] and its fork YottaDB. [1] GT.M is heavily used in production by FIS Profile banking software. (FIS was originally Fidelity's banking software division, but was spun out.) Several of GT.M's core developers left FIS and started their own company to develop the YottaDB fork. Both GT.M and YottaDB are AGPL, although earlier versions of GT.M were GPLv3, and before that GPLv2.

Another open source option is Kevin O'Kane's MUMPS implementation [2] (mostly GPLv2 or LGPL). O'Kane's MUMPS is less polished, more of a research/hobby project than production-grade code. But may be a more approachable code base if you are looking to tinker with it as opposed to using it to run a bank or a hospital. The Vista EHR (developed by US Veteran Affairs) [4] can be run on GT.M, although in production I believe VA mainly uses the proprietary InterSystems Caché. [5] As software developed by the US government, the core of Vista is public domain, although there are also a number of externally modified versions distributed under various open source licenses.

[0] https://en.wikipedia.org/wiki/GT.M

[1] https://github.com/YottaDB/YDB

[2] https://www.cs.uni.edu/~okane/

[3] https://www.hardhats.org/projects/New/InstallVistAOnGTM.html

[4] https://en.wikipedia.org/wiki/VistA

[5] https://en.wikipedia.org/wiki/InterSystems_Cach%C3%A9