I found this handy snippet to enable these keys in GTK 2 and 3 (not sure of the equivalent for GTK 4 but I guess that’s the one which has been updated anyway): https://forum.colemak.com/topic/1438-dreymars-big-bag-of-keyboard-tricks-linuxxkb-files-included/#p10012

Unfortunately I’ve found this whilst I’m not at the right computer so I haven’t been able to test them.

Edit: I tested this and it doesn’t appear to have helped.