Does this even make sense to you? I acknowledge I am not an experienced Linux user yet, but this seems most weird to me; why would KDE Plasma offer me to update GNOME?
For context, I am using Debian 13 Stable, which I installed just with KDE Plasma - so I’m not running more than one DE, nor did I install more than this.
Gnome Platform aside (as most comments suggest, probably a flatpak dependency), Discover updates everything installed through your package managers. It does not differentiate between packages. This is why it also updates GNOME Packages if they are installed