I thought the original versions of Baldur's Gate games came bundled with the enhanced editions but I can't seem to find them anywhere on my account.
Go to the BG EE (or BGII EE) page in your game library, click 'More', then 'Serial Keys' and you will find a redeem code for the original game. These will then be added to your library. Hope this helps.
EDIT: This is from the website via a browser, I'm not sure if it works the same way through Galaxy.