I assume you're rooted and/or flashed a ROM?
If you're not rooted, then you can't delete the Gallery App; you can, however, hide it from the app launcher. If you are rooted and you did delete it, I'd guess a Google search for "Galaxy Note 2 Gallery App APK" should provide answers (if any). Your other option is to restore to factory default or re-flash the custom ROM you're using (if at all).
BTW, Try to open any picture. Does it open the Gallery App? If so, it's still there, you just hid it. If it doesn't, then you're in need of a Gallery App.