The gtk and kde USE flags currently take care of installing specific polkit agents.
The gtk flag installs either polkit-gnome or lxsession, while the kde flag installs polkit-kde-agent.
This sounds fine, and is what many other ebuilds do when there's a companion program that's *likely* expected to be installed. However, in this case, I find it to be misleading.
The gtk flag is enabled by default in the desktop profile, and even in the desktop/plasma profile. This means that, on plasma, both frontends will be installed, but only one used, due to the "NotShowIn=KDE" flag in the autostart file. It makes very little sense to have both installed, as apps will only use whichever's running, and you can't have multiple running.
Different desktops have different frontends that aren't listed in *DEPENDS either, such as the new lxqt-base/lxqt-policykit, mate-extra/mate-polkit, and possibly other frontends in the future, as desktops come and go. The desktop environment meta-packages currently do a good enough job at installing the right one, and minimal window manager users will have to configure one to start at bootup manually anyway, as few of them actually read the /etc/xdg/autostart directory.
This is why I suggest removing these USE flags. It makes no sense to enable both, it depends on the desktop environment, and in the case it doesn't you have to pick one and set it up manually anyway.
I'll toss into the argument, since I've found out about it recently, that xdg-desktop-portal follows a similar implementation (one backend, multiple frontends as separate packages), and doesn't put its frontends in PDEPEND either.