Plugins:

• gerrit-trigger-plugin
• rebuilder plugin

Environment:

• CJE - 1.609.3.1 (also tested with 1.580.3 so version doesn't seem too important)
• https://github.com/fabric8io/docker-gerrit (gerrit environment - checkout and run scripts in instructions section to setup)
• gerrit-trigger.xml (attached) (gerrit-trigger-plugin config. keypath needs to change to where docker-gerrit is checked out)
• upstream job (attached)
• downstream job (attached)

Steps to create gerrit event and trigger plugin:

• Setup project (https://wiki.jenkins-ci.org/display/JENKINS/Gerrit+Trigger#GerritTrigger-SetUp) - only really needs permission for non interactive users. Call project "myproject"
• Checkout, add file, commit, then ammend it to add the Change-Id(Change-Id: I55862204ef71f69bc88c79fe2259f7cb8365699a)
• Push to gerrit - "git push http://admin@192.168.99.100:8080/myproject HEAD:refs/for/master"
• Make sure upstream and downstream jobs are loaded.
• Use "Query and Trigger Gerrit Patches" to confirm and trigger upstream job
• GOTO: downstream job and "Rebuild last"

Note that "This build is parameterized" makes you re-enter the "test" String parameter.
Also note that when rebuild happens that all the Gerrit build parameters are gone. This doesn't happen when "This buid is parameterized" is removed so suggests that this step is clobbering parameters. But not sure...

Not sure whether the issue is with:

• gerrit-trigger-plugin
• rebuilder plugin
• Jenkins core

but seems like a bug IMO.