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Is it? Having it hosted on my servers doesn't make it decentralized: it's just centralized in a place I own. If the server(s) go down, same problem as Github (except I may be able to take action). Right?

Compared to multi-tenancy it seems more decentralized to me since it dispersed functions from a central point. It is not redundant, if that is what you mean. In general having the server together with the rest of your infrastructure ensures it is available when the rest of your infrastructure is available and reduces network/ddos problems.

correction: GitLab is a centralized solution you run on your own servers - you end up with a replicated central set of hosts.

Yes, Gitlab.com is not having hosting services.

Uhhmm, GitLab.com has hosting services https://about.gitlab.com/gitlab-com/

Most people use GitLab to refer to the downloadable software you run locally https://about.gitlab.com/downloads/