Hello. I am about to download a large game. I was wondering what happens if the game gets updated while i download? Will i have to manual download it afterwards or is it added to the download right away?

Depends on how you download it. If you download it through a browser or the GOG downloader, then you'll need to either redownload the newly-added full installer or patch (depends on the game). If you use the Galaxy client to install the game, then it will perform an auto-update to the latest version after finishing its installation.