I could be mistaken, but I think the files Galaxy uses are a little bit different than the ones you can download from your browser (or with the old GOG Downloader, which still works for the time being); if that's the case, and you downloaded the game with Galaxy, then you'll probably just have to re-download the whole thing. :( If you do, I would recommend using the standalone installers, since if one part is corrupted or whatever during the DL, you won't have to download ALL of them
yet again. (The Downloader program checks the files' integrity during or after DL, and I think the newer installers all do a file integrity check prior to installation, too.)