Got to
Gamesfaq (
www.gamesfaq.com) and search there for Baldurs Gate. If you then select FAQs, there is one called AD&D- Rules FAQ. This are not the complete rules, but they give a good insight on the gamemechanics. Sorry that I don't provide the direct link to the file, but Gamesfaq doesn't allow direct linking to a file in their database.
BTW, if you are able to get the 2nd Ed. AD&D rules, be aware, that Bioware changed some rules and spells slightly. Especially in BG2 there are some concepts from the 3rd Ed. rules included, like the sorceror or the babarian.