I experience the same as Julien yesterday in a computer reconfigured with sway (no GNOME), but still using GDM. Here's a screenshot of "top" with gdm's .gnome-shell-real using 2.5 GiB of RAM:
https://multimedialib.files.wordpress.com/2020/03/gnome-shell-real-leak-sway-2020-03-22.png I'm currently trying other other desktop environments in combination with slim login manager to work around the issue.
