Pretty close to the same at least. The main distinction would be that the Steam version still requires a copy of Steam to be running and logged in on the computer you copy it to, which at least means Steam has to have been online once ever to get the account logged in before using offline mode. GOG has offline installers that can be backed up and used without any client.
For the vast majority of use cases, it’s a pretty minor difference, but one way in which it might be significant is that the GOG installers will never stop working, but if one day years down the road Steam were to shut down, the Steam version could only run on computers that could be running offline-mode Steam. There’d probably be ways to break that simple bit of DRM, but a legal offline installer is a very nice bonus for things like archival sites or research applications.
It’s the kind of thing that even if you’re not choosing to use it, it’s nice that it exists, and hopefully it can continue to.