Hello Mohamed,
The gitlab-oauth-plugin was forked from the github-oauth-plugin.

I don't mind that the code was forked and reused (in fact the license was expressly designed for it). However, when forking the only stipulation is that you keep the license so others know where that code was obtained.

Please honor https://github.com/jenkinsci/github-oauth-plugin/blob/master/LICENSE.txt and simply append your own copyright to the bottom of the list of copyrights.