You can already do soft proofing and use ICC profiles in GIMP. There are also plugins. And seems like more CMYK features are under development.

https://docs.gimp.org/3.2/en/gimp-prefs-color-management.html

https://wiki.archlinux.org/title/GIMP/CMYK_support

https://www.phoronix.com/news/GIMP-Post-3.2-Features

https://developer.gimp.org/core/roadmap/

3.4: High-end CMYK support