It's not trivial. Just installing KDE packages on a GNOME install will work and is quite easy, but will lead to some mix / subtle setting issues, it's less clean than just a brand new install.
Installing and running KDE will mess up GTK settings in GNOME for instance. You might end up with the Breeze GTK theme in the GNOME session. Which works, but this is most likely not wanted (even though GNOME looks great with the Breeze theme).
I'd not advise regular users to do this without a warning.
I haven't seen this on my Linux laptop, but TBH some UI elements in GNOME look so weird in Ubuntu 24 that I'm not sure if it's broken or intended (but already did before installing KDE).
Installing and running KDE will mess up GTK settings in GNOME for instance. You might end up with the Breeze GTK theme in the GNOME session. Which works, but this is most likely not wanted (even though GNOME looks great with the Breeze theme).
I'd not advise regular users to do this without a warning.