CosmicVoyagerX: DRM is copyright protection.
Those two terms overlap but are not 100% interchangeable, particularly since the meaning of "DRM" has changed. Eg, older retail games were copy protected (often with a serial or code-wheel), but not neccessarily "DRM'd" in the sense you could legally resell them on EBay and they would work exactly the same way for the new buyer. Modern DRM typically means tying games to a person at the account level and preventing any resale via constant online checks long after you've bought them. GOG installers can't be resold, but at the same time, once you have them, the installers themselves do not make any checks at any point, so in that respect there's no copy protection or DRM in any practical sense once you have them.
Personally, I wouldn't call the serials that come with some games that seem to be required solely for 3rd party multi-player servers as "DRM" as the underlying purpose is often just to provide a unique identifier for the server to distinguish between gamers (particularly for any persistent worlds). Same goes for serials for other purposes, eg, Don't Starve comes with serials for the developer's own update path and I've never used them once (nor are they ever required), because GOG updates their own version anyway.