There is no strict policy in wayland to disallow root applications. KDE Partition Manager runs fine as wayland client on wayland.
That said, it is still not a good idea to run GUI as root on either Wayland or X11. It's actually worse for X11 since other applications can use X11 protocol to do malicious stuff via GParted. Wayland protocol is more secure. So right now KDE Partition Manager is migrating to Polkit/KAuth helpers, so that GUI can run as unprivileged user. GParted should do the same.
