In the Atari ST world GEM is still an existing and maintained thing, open source and somewhat modernized.

In the forked and (heavily) maintained form of EmuTOS (GPL, https://emutos.sourceforge.io/)

Or in the from-scratch rewrite in the form of MyAES http://myaes.lutece.net/ (AES ["application environment services"] is the GUI & messaging layer of GEM, sitting along side VDI, the graphics layer)