Earlier quoted context omitted.
Right, I just realised the GitLab Web IDE is running fully in the browser, so there is no Terminal. Maybe they are considering a later version which is more like Codespaces, letting you develop and run your code, keeping the browser as the user interface.
https://docs.gitlab.com/ee/user/project/web_ide_beta/#known-...
> The Web Terminal and Live Preview are not available in the Web IDE Beta.
> These features may become available at a later date.