Unfortunately there is no specific definition of DRM. GOG is right if the meaning is: "You can copy and use the version freely, without any CD check or product activation."
Yes, copy protection is one type of DRM, but it can still have DRM even if it doesn't have copy protection.
"Product activation" You can't use your GOG download license without an account, download license, and authorization from their server. Some old physical copies with offline DRM such as offline keys and disc checks never required online DRM such as an account, download license, or online authorization unlike GOG and you could even copy and paste the files after activation and play some of them without ever having to go through the offline DRM again, and you can sell them and the buyer can use it just as much as the initial purchaser without the developer or publisher ever knowing despite it having DRM. GOG download licenses can be easily and legally revoked, old physical copies, even those with offline DRM cannot.
They claimed the day 1 "physical" Witcher 3 release for PC was somehow DRM free even though you need a download from GOG servers to be able to install the game from the discs. That physical release isn't DRM free despite what they said, but they inadvertently admit that copy protection isn't the only type of DRM despite what they claim.
GOG uses that incomplete definition to make themselves look better when in reality they aren't any better than the other scumbags and to attack physical releases.
Games with Denuvo are not DRM free even though after they are cracked they can be copy and pasted and played without ever having to be authorized.
CDPR/GOG don't care about being DRM free as proven by the fact that they have single-handedly changed the perception of what is and isn't DRM in many video game players' minds and the day one "physical" releases for PC of all three Witcher games have had DRM, as will Cyberpunk 2077 as it is already confined to be a download code which is inherently DRM. Before you give the typical digital distribution pay pig straw man response of "but no one has disc drives anymore" - CDPR/GOG have helped to make it that way with their brainwashing, optical media is the king of data storage for longevity despite what the video game and digital distribution industries want you to think, external drives exist and while being more expensive than internal drives, you can get a BDXL and M-disc compatible external drive for less than what a download license of Witcher 3 cost on release day and the two expansion and they can be moved from build to build, and they could always use some of their money to develop a "USB ROM" for PC, a USB drive that uses ROM instead of shitty flash memory, they have plenty of money to do so, but alas, that isn't as profitable and if they did that and more people went with that instead of paying for a download license which is what would happen if it got even a small fraction of the push that digital got then they wouldn't have as many people to harvest data from (which GOG and every other digital distributor does), be encouraged to release games finished and have no way to easily shill people DLC since fewer people would be connected to some platform and so on, and they prefer to release their games unfinished anyways because more $ with less effort and have tricked people into thinking expansions and """""free DLC""""" are a good thing when all that means is the game was released unfinished.
My definition of DRM is anything intentionally added in to the mix that doesn't have to be there to prevent unauthorized use of the original source, the server for digital and physical media for physical. GOG doesn't have to require an account, download license, or authorization to download from their servers just as those old physical copies didn't have to have keys, disc checks, or securom. You could have took a laptop into a store and installed and played a game from one of those old physical copies without paying, and if someone bought that opened copy they would have been able to use it just fine, same as the person that opened it, and so would the person that bought it from the initial purchaser if they ever decided to sell it. Of course CDPR/GOG could have always called for physical copies to be DRM free but that isn't as profitable, and they and Valve wouldn't be able to use "no keys" as a marketing tactic (which is ironic because Valve's download license system uses keys and I assume GOG does too) even though those keys never needed to be there to begin with, if they didn't want keys then they could have stopped adding them to the physical releases.
People are way too lenient on the video game industry. They are companies, not your friends, just because what they make is fun doesn't change that.