octavius Apparently, "AD&D 2.5" is actually a thing, and both BG1 and BG2 use it
http://en.wikipedia.org/wiki/Editions_of_Dungeons_&_Dragons#Differences_from_Advanced_Dungeons_.26_Dragons_2nd_edition
In 1995, the core rulebooks were slightly revised and a series of Player's Option manuals were released as "optional core rulebooks". Although still referred to by TSR (and later Wizards of the Coast) as the 2nd edition, this revision is seen by some fans as a distinct edition of the game and is sometimes referred to as AD&D 2.5.
So Gold Box is AD&D 2, Infinity Engine is 2.5. What I wonder is whether there was ever an AD&D first edition game. It'd have to be unofficial since I don't think TSR licensed out before SSI.
None of the IE games actually use the "player's option" books (most memorable thing in them is that they split up each attribute into two parts, like Dexterity gets split up into agility and... I think hand-eye coordination but that isn't the word they used). If a video game ever used that shit it must have been something I've never played.
BG2 just took a few kits from "Complete Book of X", which isn't the player's option stuff. Maybe Icewind Dale did Complete Book of X also but that game has passed through my memory leaving no trace
Also the biggest video game thing for AD&D 1st edition is that you get experience points for loot, which is in all the Gold Box games. GBs also don't do shit like weapon specialization which was a rule from some expansion book in 1st edition (Unearthed Arcana?) but a rule in the core book in 2nd.