Andrea Veri commented:
@thibaultamartin that really depends on how you want the website domain to be visualized, a plain redirect between gnome.org/os and the gitlab pages URL is just going to represent itself in your browser by referencing the final URL aka https://bertob.pages.gitlab.gnome.org/gnome-os-site. For a sub-domain we can actually hide the final URL into i.e os.gnome.org. We can also redirect gnome.org/os to a sub-domain if that's what you want.