Hardrada: I expect we will see the first few here eventually. Since Empire the Total War games have used Steamworks so are less likely to make it to GOG.
What is the Steamworks used for in those games?
E.g. Rome 2 Hannibal at the Gates (with all the DLCs) and Attila seem to be readily available on some "unofficial" sites (just checked it), and the crack team installation instructions don't mention anything about needing a Steam client.
Multiplayer is the thing that might me missing from the GOG version (unless it is moved to Galaxy), but then that is nothing new, there are lots of GOG games with the multiplayer component being non-functional.
Anyway, I would expect the older games appear first, if they came here. GOG could start by re-releasing the Eras collection.