As GamersGate does not want to be seen like a DRM (even it is a light one*, for being easily hacked), they changed the definition of DRM:
http://www.gamersgate.com/supportcenter/index.php?/Knowledgebase/Article/View/141/171/what-is-what
That's why for GamersGate a simple serial key is an "Other DRM" (like GOG is a DRM for the Steam fanboyz), to make GamersGate appear less a bad DRM.
Yes, a serial key is a digital right management as tell the words (you don't enter an iron key in a lock of the DVD box), but it is not an enforced one like the
true usual DRM which needs a 3rd party compelled hardware (often a distant server, like Securom or Steam) or compelled software (like the hacking drivers of Starforce).
A battle for a bad word.
* You're told needing to enter the GamersGate servers each time you install a game.
GamersGate know their DRM is easily hacked, and knows that many do buy there for that...