We have a number of jobs triggered by gitlab and thus we use the gitlabSourceBranch and gitlabTargetBranch variable to get the correct branch from gitlab.
To allow manual triggers we enable the "this job is parameterized" option and set 2 string parameters to some default value and with the same names gitlabSourceBranch and gitlabTargetBranch. Till before about 1 week the job parameters default values where overwritten by the gitlab hook values, but after some updates it's the other way around, so the gitlab hook values are overwritten by the jobs default values...