Hi,
as @jan already mentioned we are aware that the versioning of our software is difficult to understand from the outside. We have already started to improve the situation. First of all we switched to time-based releases where we only have major and bugfix releases.
The GVM releases will be done approximately one month before each new GOS release. Thus there will be no GOS 5.1 nor GVM 10.1.
GOS 4.x is GVM 9
GOS 5 will be based on GVM 10
GOS 6 will be based on GVM 11