Small update here: Gnome 40 (the next version) will heavily extend its usage of gestures and will use three-finger swipes to left/right for switching workspaces.
In https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/3528#note_999447 I was given the advice that we should use two-finger swipes for going forward/backward. That would match Epiphany/WebKitGTK as well as Firefox on MacOS behaviour, thus is probably the best way to go forward here.
IIUC what we'd need to do is to implement a swipe tracker for the Wayland backend (not sure if X11 is also feasible - wouldn't want to spent any extra time on it). So there places to look at would be
widget/cocoa/SwipeTracker.mm and Epiphany. I hope I/we will find some time to look into that in the coming month so things are in place for the spring distributions.