When I run xkill in terminal a X (cross sign) appears which seems to kill that GUI process (application) on which it is hovered and clicked. I expect the same behavior for GNOME Terminal (since in my understanding it is also a GUI application). But I get different behaviors under different display manager.
Under x11:
Under Wayland:
It seems xkill can't kill terminal in Wayland.
According to a bug report and response by Jean-Batiste Lallement:
This is a known issue with wayland documented upstream on https://fedoraproject.org/wiki/How_to_debug_Wayland_problems#Many_well-known_X11_utilities_don.27t_work
Don't hesitate to file any bug you may find.
To quote Fedora documentation:
Power users are familiar with a large range of X11-related utilities, like xkill, xrandr, xdotool, xsel. These tools won’t work under Wayland session, or will only work with XWayland applications but not Wayland applications. Some tools might have a replacement which allows to perform similar tasks.
Thus, it's a well known issue. You may want to submit a bug report (and probably it will be marked a duplicate) or wait until upstream fixes things.
For the time being, you may want to stick with the pkill or kill in terminal
This is by design of Wayland.
As you are using Wayland, this is expected that you can not use X11 tool named xkill to kill a client by its X resource (part of x11-utils package) here.
Also you can not use xdotool, xinput (that is good as you will have more secure GUI) here and others.
In my case (Ubuntu 22.10, Gnome, Wayland) this worked for crashed and not-yet-crashed windows:
press Alt+F4
like described here: Kill a specific window, but not the entire suite of windows belonging to one program?
