First, the TW2 Linux port on Steam isn't a true port as it still used the Windows binaries with the eON wrapper, much like running TW2 with Wine. The current state of the port on Steam is very early, it has a number of performance issues and problems. From what has been said about this before, GOG will most likely get the Linux version once the performance issues and bugs are minimized, when that will be is anyone's guess.
About the GOG downloader, there currently is an open source Linux downloader provided by the community, more info can be found here:
http://www.gog.com/forum/general/lgogdownloader_gogdownloader_for_linux/page1 GOG is working on a new client called GOG Galaxy and I think it should have a Linux version, more info can be found here:
http://www.gog.com/galaxy