CHANGE_TARGET is intended to represent the "target branch" of a change request – i.e., the target branch that the change will be merged into if approved and submitted. Currently this gets set to the change ref. These variables were printed using sh "env".

// git push gerrit HEAD:refs/for/master
CHANGE_TITLE=53/106353/19
CHANGE_TARGET=53/106353/19
CHANGE_ID=C-106353/19
CHANGE_URL=https://gerrit.server/106353
GERRIT_BRANCH=master

In this case, GERRIT_BRANCH represents the target branch, but because it's not set in CHANGE_TARGET, we cannot use the changeRequest conditional in a declarative pipeline:

pipeline {
  stage("for master only") {
    when {
      changeRequest target: 'master'  // will not work
    }
  }
}