I haven’t got around to KDE 6 yet, but have you tried installing gamescope from the Extra repo instead? (since that has been updated more recently; just 2 days ago)
libdisplay-info is needed for gamescope. The git version of gamescope might need some updating to fix this issue.
https://archlinux.org/packages/extra/x86_64/libdisplay-info/
It’s also needed for kwin, so that might of been why there was difficulties with the KDE6 update.