Postal 1 Steam Vs Postal 1 Gog

Is there a difference with the Gog version as it is labeled "POSTAL: Classic and Uncut" Does that mean it has more things that Steam wouldent allow or something?I am hoping to pick this game up I just played Redux and I wanna play the originalOne more thing, Sense this game is open sorce, Does that mean you could just build the game, play it and edit it as you want? Do you have to buy the game to edit the source code? I'm not saying I would pirate the game or anything I was just wondering how you would be able to obtain it.As always thanks RWS