Its not possible to access gitlab variables:

Example:

• gitlabBranch
• ...
• gitlabMergeRequestId
• ...

Multibranch Pipelines expose additional information about the branch being built through the env global variable, such as:
With the "GitHub" or "Bitbucket" Branch Sources, Multibranch Pipelines can be used for validating pull/change requests. 
CHANGE_ID - An identifier corresponding to some kind of change request, such as a pull request number

Please add the same functionality to Gitlab plugin.