When you specify %t in the log file for garbage collection:
Xloggc:$path/gc%t.log: specifies where the verbose GC data is written to. The %t in the gc log file path will cause the JVM to generate a new file with each JVM restart with a timestamp.
From: https://go.cloudbees.com/docs/solutions/jvm-troubleshooting/

The support-core plugin cannot find the log file used for that, so it doesn't include the file in the bundle. It throws the warning:

Jun 24, 2019 3:42:25 PM com.cloudbees.jenkins.support.impl.GCLogs addContents
WARNING: [Support Bundle] GC Logging apparently configured, but file '/JENKINS HOME PATH/gc-%t.log' not found

The reason is that the code looks for the exact file name, but it should be a pattern:
https://github.com/jenkinsci/support-core-plugin/blob/5fb61461e578fa82190c8a1f1dd5efd95235702f/src/main/java/com/cloudbees/jenkins/support/impl/GCLogs.java#L80