Opening up Konsole and entering:
sudo pacman -S gamescope
should get you gamescope from the Extra repo. Be warned that there has been a report from one user saying that version was somehow messed up their Steam. (I have not verified this yet, so there could be something else at play here) Gamescope-git conflicting files
I will also link the wiki page about snapshots here, just in case things really break and you ever need to rollback to a prior snapshot: