Monitoring > Requests Profiles admin page (highlighted in a blue in the image below). 
Pass the header X-Profile-Token: <token> to the request you want to profile. You can use any of these tools
curl --header 'X-Profile-Token: <token>' https://gitlab.example.com/group/projectOnce request is finished (which will take a little longer than usual), you can view the profiling output from Monitoring > Requests Profiles admin page. 
Profiling output will be cleared out every day via a Sidekiq worker.