Well, you can keep Gnome. To install Plasma just do this:

sudo pacman -S plasma
and if you want the full KDE apps, install the KDE-applications as well or pick the apps you want from this group. (konsole, kate, etc.)

As for default DE, you can just select Plasma from GDM login screen (assuming you are using GDM with Gnome) or just append exec startkde to your .xinitrc then do startx.
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