Comment by Rochus

The most obvious alternative is https://sourceforge.net/projects/fis-gtm/, but there are a few others, depending on the size of the application at hand. https://yottadb.com/ is another big one which started on GT.M a few years ago, or http://minimdb.com/. See also https://www.mumps.dev/implementations.cfm.

If you don't depend on the MUMPS/ANSI-M language or don't have to be backwards compatible, a lot more alternatives with similar features are available, e.g. https://github.com/facebook/rocksdb run behind e.g. Node.js.