EDIT: It's been like that since at least 12h ago though. Not sure if it's connected to Google Cloud?
GitLab is no longer seeing errors and Google Cloud has resolved the issue as of 23:00 UTC yesterday. Any further information can be found on the issue at https://gitlab.com/gitlab-com/gl-infra/production/issues/862
(the problems with runners and the UI started at least at 2019-06-02 7:48 UTC, though they were hit-and-miss at the time)
Still, happy this is solved and we can use the (fantastic otherwise) service again!
EDIT: It's been like that since at least 12h ago though. Not sure if it's connected to Google Cloud?