There are many integrations for GitLab available, but one very interesting one is Mattermost, a Slack-like chat collaboration tool. It was acquired by GitLab in 2017, and is now even integrated in the omnibus GitLab install. You can enable the ChatOps operation with this tool. In the settings, you can enable the specific button in GitLab:

 mattermost:
    enabled: false
    host: 'https://mattermost.example.com'

By enabling this, you can use the Add button in GitLab in the service settings.