Zendesk is the more common of the two in our index: it runs on 4,144 websites against 5 for GitLab, and 2 sites use both. Counts come from real scans, not vendor claims.
GitLab is a web-based DevOps lifecycle tool that provides a Git-repository manager providing wiki, issue-tracking and continuous integration and deployment pipeline features, using an open-source license.
Zendesk is a cloud-based help desk management solution offering customizable tools to build customer service portal, knowledge base and online communities.
2 sites run both GitLab and Zendesk. That is 40% of the sites using GitLab, the smaller of the two.
Want to know which one a particular site uses? Look it up and see its whole stack, or compare two sites side by side.