While I would like to take credit for this concise chronology of DDK's appearances on AtS, I cannot. It is copied word for word from DDK's official site, and the webmast/ress at the site is responsible. To thank her for her work, mail it!
Episode: "Over the Rainbow" (episode # 2.42) (5/8/2001) Gavin introduces himself to Angel and company when he comes to appraise the hotel.
Episode: "That Vision Thing" (episode # 3.46) (10/01/2001) Gavin renews his campaign to "paperwork" Angel to death.
Episode: "Carpe Noctem" (episode # 3.48) (10/15/2001) Gavin engages in some office politics with his co-worker Lilah.
Episode: "Billy" (episode # 3.50) (10/29/2001) Gavin is infected by a woman-hating demon and gleefully throws Lilah against a wall then tries to choke her.
Episode: "Quickening" (episode # 3.52) (11/12/2001) Gavin's surveillance of Angel reveals shocking news that sends Wolfram & Hart into a frenzy. Gavin has a lot of screen time as he and Lilah maneuver to gain the most in the chaos.
Episode: "Lullaby" (episode # 3.53) (11/19/2001) Gavin, Lilah, and their latest human boss try to determine what happened at Angel's place when the lights went out.
Episode: "Dad" (episode # 3.54) (12/12/2001) Picking up right after "Lullaby", Gavin continues to conspire, undermine, curry favor and look sharp in a suit - so a normal day at the office.
Episode: "The Price" (episode # 3.63) (04/29/2002) After an absence, Gavin is back to harass Lilah, trying to get her to make a wrong move in dealing with Angel's latest disaster.
Episode: "Tomorrow" (episode # 3.66) (05/22/2002) Gavin has a brief appearance, watching his commandoes try to kill Angel and then watching Angel threaten his boss.
Episode: "Deep Down" (episode #4.67) (10/06/2002) Gavin's fortunes change significantly when Lilah goes from his co-worker to his boss.
Episode: "Rain of Fire" aka "Apocalypse, Nowish" (episode #4.73) (11/17/2002) Gavin makes an extremely brief appearance as a pitiful silent bundle.
Episode: "Habeus Corpses" (episode #4.74) (1/15/2003) Gavin tries to save Wolfram & Hart and himself from The Beast, and fails to do either. He is killed by the beast, but rises as a zombie to fight Angel and his friends.