- Unzip the bundle (target/nexus-gitlab-token-auth-plugin-0.4.0-SNAPSHOT-bundle.zip) under sonatype-work/nexus/plugin-repository
 - Copy & Edit the 
sample/gitlab-plugin.xmlto:sonatype-work/nexus/conf/gitlab-plugin.xml - Start Nexus and use the Administration->Server panel to add Gitlab Token Authentication Realm to the list of active realms.
 
 <?xml version="1.0" encoding="UTF-8"?>
 <gitlabConfiguration>
  <gitlabServerUrl>https://git.nexgen.neustar.biz</gitlabServerUrl>
  <defaultRoles>
    <defaultRole>person</defaultRole>
  </defaultRoles>
  <adminRoles>
    <adminRole>nx-admin</adminRole>
  </adminRoles>
 </gitlabConfiguration>
First: mvn generate-sources
This generates Configuration stubs
Then you can make changes using the configuration classes.
Build:  mvn clean install -DskipTests=true
Generates: target/nexus-gitlab-token-auth-plugin-0.4.0-SNAPSHOT-bundle.zip
Integration Tests: mvn clean integration-test -P it
Now you can run Integration Tests: mvn test
Thanks to the Crowd Plugin: https://github.com/PatrickRoumanoff/nexus-crowd-plugin This was heavily influenced and adapted for Gitlab.