The following are recommendations for your new Guix system. Some are nonfree, these are indicated with a bold NF next to the heading.
More likely than not, you want to install a window manager or desktop environment. KDE is not installable on Guix due to numerous issues, XFCE would be a good alternative to look at.
Quite a few window managers have built in support:
There are also desktop environments for those who prefer them: