The following GC Phase events now have eventThread, added by Stefan Karlsson

Attribute "eventThread" in event metadata for "com.oracle.jdk.GCPhaseConcurrent" was not found in baseline recording
Attribute "eventThread" in event metadata for "com.oracle.jdk.GCPhasePause" was not found in baseline recording
Attribute "eventThread" in event metadata for "com.oracle.jdk.GCPhasePauseLevel1" was not found in baseline recording
Attribute "eventThread" in event metadata for "com.oracle.jdk.GCPhasePauseLevel2" was not found in baseline recording
Attribute "eventThread" in event metadata for "com.oracle.jdk.GCPhasePauseLevel3" was not found in baseline recording

It would make sense to display these threads on the GC Page, for example with a thread chart like on Java Application page, but only for the threads where these events occur.
(Not sure how to find out which those threads are, except for filtering on the event types first, and then extracting the threads)