Comment # 5 on bug 931314 from Dominique Leuenberger

sddm set XDG_CURRENT_DESKTOP based on the Entry DesktopNames in the session's
desktop file.

This was specified at the freedesktop summit in 2014, documented in November
2014 as part of the desktop specification.

Seems the other DEs are falling back to using Name= if DesktopNames is not set;
SDDM simply expects the right key to be there.

GNOME Upstream actually fixed that already with GNOME 3.12.1 in April 2014
(shortly after the freedesktop summit)

in GNOME, though, we have an own gnome session .desktop file for proper
integration into the rest of the distribution - and this addition was lost.