jenkins credentials not used as expected when cloning git repo
The Gitlab plugin requires an auth token, you're trying to use an SSH key.

https://github.com/jenkinsci/gitlab-plugin#global-plugin-configuration

"PLEASE NOTE: This auth configuration is only used for accessing the GitLab API for sending build status to GitLab. It is not used for cloning git repos. The credentials for cloning (usually SSH credentials) should be configured separately, in the git plugin."