How does an offline installer from GOG differ from the offline installer provided on a CD/DVD?
The license for the DVD version is with the actual disk, the license for the offline installer is with the GOG account.
GOG has essentially created a way to bypass their own licenses, as a feature. And it looks like they won’t be affected by this law because of it.
They haven’t created anything.
They just don’t allow games that use DRM (any kind of license check as a prerequisite to run software) on their store. Packaging a game with DRM is an extra step.