@codayon Thanks for the quick answer!
And I think I will give it a try anyway, because so I can test the setup a little bit. Installing fresh is possible anyway later! 
What do you guys think would be the best way:
sudo systemctl disable sddm.service
sudo systemctl enable gdm.service
sudo vim /usr/share/xdg-desktop-portal/portals.conf
--> change to:
[preferred]
default=gnome;kde;
sudo pacman -Syu
sudo pacman -S gnome gnome-extra
sudo pacman -S xdg-desktop-portal-gnome
sudo reboot