DebugMode: Steam is a client. Steamworks is DRM.
Steam has several levels of DRM (from least to most severe):
1. No installers provided for games (unless you can manually back up the game directory to transfer to another computer, which is not always possible.
2. Requirement to run the client to play some games (steamworks).
3. In game DRMs (on-line requirements and etc).
All of those are DRM, and GOG has none of these. 2 and 3 aren't always present in Steam, but I think 1 is always there.
shaddim: I hope not, they should allow direct placing of update and patches in good old self-contained form ("exe") for developers.
Should be placed in the game's download directory as additional patch (no overwriting allowed) with version +1.
Whatever that would be - a single download, or a series of patches and etc. As far as it will be provided as a file that can be backed up DRM free, it's good enough. So GOG can combine both - convenience of incremental updates through their service, and an option of DRM free backups for installation.
I'd just add, that if GOG is to create some client for their service, it better be open source, in order to be fully trusted.