1. The plugin https://wiki.jenkins-ci.org/display/JENKINS/GitLab+Plugin is installed, but invisible or not available to be configured at "Build Triggers" when the multi-branch job is configured.
If we go to the created job for a particular branch the option is available and could be configured.
2. On each reindexing on multi-branch job, the previously configured Gitlab settings for the branch jobs are reset and Gitlab build trigger is disabled.