...Over the web...
I mean, Git and Linux are developed via email lists, which is a decentralized way of sending pull/merge requests, isn't it? I guess you could argue the mailing list is hosted on a server, fine. So then fine, usenet.
Yeah, email and nntp are old crufty technologies and there are obviously advantages to having a web based interface and a central place to go for a project. But git itself certainly supports a decentralized mechanism for merging.
So then the question really is, how do you decentralize a web interface, isn't it? The dvcs itself isn't the problem.
That should be possible via browser support for site-based URL handlers; for browsers without such support, you could also have a URL on the main GitLab site that people can register with to redirect to their "home" instance.