Bit late to the thread. The GOG versions are definitely the remastered versions.
As for what "remastered" means. Legacy is the only one that had anything major done to it (new voice acting, plus an additional "five years later" commentary track). The only updates to Unbound and Convergence were that they were recompiled in the latest version of the AGS engine. Some minor code was written to ensure compatibility, but nothing very noticeable when compared to the original versions.