David Gilmour's first full album with Pink Floyd is "A Saucerful of Secrets.

Yes, that’s the one. Not “The Piper at the Gates of Dawn” — he played a little on that as a session guitarist, but he wasn’t a member yet. By “A Saucerful of Secrets,” he’d officially replaced Syd Barrett. So if you’re looking for the first album where he’s a full-time band member, that’s it.