It's not the entire DE. You have application installed made for Gnome and that is one of its dependencies.

Don't worry, I get KDE updates in GNOME.

That is a runtime that some flatpaks use as well.

Gnome and KDE as projects are a bunch of things, from login managers, to compositers, desktop UIs, and user application (like Gnome "System Monitor" or KDEs "Plasma System Monitor").

You can actually mix and match some pieces and they just work, but especially the user apps because both teams put in work for interoperabilty or Freedesktop standarization.

So you can have an app that uses KDEs shared libs and an app that uses the Gnome projects libs on the same issue with rarely any issue. Even more so with flatpak since the all of the files those apps see are in what is called an overlay filesystem, so your kde apps get a layer of files jist for KDE apps to build off of and gnome apps get a layer of files just for them to use. In flatpak these are called runtimes. That is what is being updated here.

All the "App Store" apps like Discover are merely frontends for your system's underlying package manager (apt for Debian and derivatives, dnf for Fedora and its derivatives).

The underlying package manager does the updating of packages: if you've installed it through the package manager (which is usually most stuff on an install) - it'll get updated.

Discover just gives you a nice, user-friendly way of interfacing with the package manager(s) on your system so you don't need to bother with the CLI if you don't want to (that's what "frontend" means - a nice, friendly UI for underlying services).

And yes, you can have multiple - for example apt and Flatpak. Discover and friends should update all.

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

The GNOME platform application is used by flatpaks. Basically, a flatpak can be built against/designed to be used with a specific visual toolkit. To do that, it needs to download specific parts of that toolkit, which is what you're seeing.

Looks like you've installed a package with flatpak that depends on GNOME

It always surprised me but I never bothered asking why.

Thanks for doing it and thanks to people providing an explanation.