ZGC is omitted from the TOC of this 'HotSpot Tuning Guide' chapter. Note that the ZGC section is present, but just missing from the TOC.
https://docs.oracle.com/en/java/javase/15/gctuning/available-collectors.html#GUID-F215A508-9E58-40B4-90A5-74E29BF3BD3C
https://docs.oracle.com/en/java/javase/15/gctuning/available-collectors.html#GUID-F215A508-9E58-40B4-90A5-74E29BF3BD3C