I have recently installed Mountain Lion (clean install) and my gate keeper is set to "Mac App Store". I found an old app downloaded on my mac, it was a game named Tribbi.app when I tried to open it, It just opened, Gate Keeper Didn't prevent it. This app was downloaded in 2010 so it was before gate keeper or Mac App Store and I had never opened it before on mountain lion, I was wondering how is it even possible?
According to this Macworld article, Gatekeeper checks only apply to applications downloaded from the Internet (and by that, they really mean apps downloaded via Safari, Mail, Chat, and other selected apps or apps which use common frameworks). According to the article, the underlying implementation still relies on the File Quarantine system from 10.5.
Based on your description, the app wasn't quarantined on the system and wasn't moved to the new system through an application that triggers a Gatekeeper check, and that's why you don't have any problems opening it.