you run the installer that gog make for each game. the backup should be placed on a separate drive preferably 1 you dont constantly write to to reduce the chance of file corruption. other than that you should ave no issues installing the game from the files you downloaded. gog do a lot of re-releases all of which are drm free. i recommend them highly, $5-$10 for classics is a bargain. especially when they can appear on steam and origin at more than x2 the price with there drm intact.