When GitLab sends a push notification to Jenkins via gitlab-hook's endpoint: /gitlab/build_now, the build cause message (e.g. "Started by user so and so, Started by GitHub push by so and so, Started by timer", etc...) is set in HTML by gitlab-hook. I had an issue over there, but they told me to make one here as it is Jenkins job to parse this HTML and not theirs.
This is what I see in plain text when a build runs via GitLab hook:
"Started by remote host git.redacted.com with note: <br/><br/>triggered by push on branch refs/heads/redacted<br/>with 1 commits:<br/>* <a href="redacted">Redacted</a>"
Instead of what should be seen (view attachment for what should be displayed or click here: https://cloud.githubusercontent.com/assets/674883/3699700/75ddbb34-13d7-11e4-99a2-483bc77e3ee4.png)