For GOS 6.0, GOS 5.0, and GOS 4.3 new patch levels are available:
https://www.greenbone.net/en/roadmap-lifecycle/
These releases include security improvements for all versions, as well as various fixes for outstanding issues in GOS 5.0 and GOS 6.0. Please refer to the Roadmap & Lifecycle page for a full list of changes.
In total GOS 6.0.10 covers 13 improvements, GOS 5.0.19 covers 12 improvements and GOS 4.3.21 covers 2 improvements.