GitLab plugin creates a "Use custom workspace" configuration option that does not write to file on save/apply. 

Reproduction steps:

• Create a new Jenkins instance
• Download GitLab Plugin and restart
• Create a Freestyle Project
• Select "Use custom workspace" above "GitLab Connection" and define a path
• Click Save
• Check Configuration or config.xml and see that custom workspace path was not saved

Workaround: use "Use custom workspace" option under Advanced in General section in Job Configuration.