I would wager that your self-hosted GitLab has less uptime than GitHub (or the quality of the deployment). We typically underestimate the downtime of the software we deploy ourselves, probably because we're attached to it, or because we're busy fixing it, instead of having to wait.
I'd take the occasional downtimes of cloud solutions, if it means not having to use some of the self-hosted software I had to to use at work.
I'd take the occasional downtimes of cloud solutions, if it means not having to use some of the self-hosted software I had to to use at work.