Gideon has been (unofficially) working for the CIA.

Apparently, Garcia is on some sort of CIA "watchlist" and is not allowed to enter Langley as a result.

Gideon: (as the team study the photos of John Summers) CIA made it look like suicide.
Reid: I'd like to have that job.

Hotch: How long have you unofficially been working for CIA?
Gideon: Long enough to know whom and what we're dealing with.

Bruno Hawks: It was only the people in my unit that knew that John Summers was CIA. And my bosses believe that someone in my team is the one that had him killed. We want to know who and why.
Gideon: A mole in Langley.
Bruno Hawks: It looks that way and I'm not high enough on the food chain to dodge the bullet on this. (Gideon picks up the phone) No, no, Jason, no. You know the rules. No judge, no jury, this is strictly in-house.
Gideon: You want my help?
Bruno Hawks: Yeah.
Gideon: I need my team.

Gideon: Albert Einstein said: "Whoever undertakes to set himself up as judge in the field of truth and knowledge is shipwrecked by the laughter of the gods."

Garcia: We have a CIA escort, okay? It's weird, it's scary.
JJ: What do you suppose this is all about?
Reid: Call me cynical but considering it's 2am, I doubt it's good news.
JJ: You are a genius.

Garcia: The thing that I don't get is our team's there, why can't we go to Langley? Do I look like a threat to national security to you, seriously?
Agent #1: Your name is on a list.
Garcia: A list? What list? I work for the FBI and you're telling me I'm on a list that says I can't go to the CIA?
Agent #1: You're on a list.

JJ: 21 and 22 are here with us... in the room.
Garcia: What if it's one of them?

Gideon: And we have to assume the mole has an exit strategy.
Hotch: Only now we're blocking the exit. And the only way out is through us.

Gideon: It's good to see you again.
Gina Sanchez: You're not a very good liar.
Gideon: I wasn't lying.
Gina Sanchez: Your evaluation took me out of the field.
Gideon: I simply wrote the truth. You're highly intelligent, you're impossibly strong-willed and there's gonna come a time when you can't follow orders.
Gina Sanchez: It hasn't happened yet.
Gideon: It will.
Gina Sanchez: You're wrong.

Gideon: One thing I've learned over the years profiling CIA agents... spies are some of the smartest liars in the world. Be smarter.

Kruger Spence: Alright, start looking at where Aaliyah would feel the most comfortable. Arab-populated areas...
Reid: (interrupting) Actually, you might want to start by profiling Summers.
Kruger Spence: Be my guest. Aaliyah's about to be brutally murdered in front of her children. Got nothing? Okay. Where were we? Alright, let's put some teams together...
Reid: (interrupting again) Summers was obsessed with his own security.
Kruger Spence: Aren't we all?

Kruger Spence: Look, if you want to arrest us for having an affair, be my guest.

Kruger Spence: Look, people don't just get murdered inside the CIA!

Gideon: (to Bruno Hawks) Who says, you know, kids need to know everything about us? Trust me, we're not that interesting.

Hotch: (after Garcia figures out the whereabouts of Aaliyah) Garcia, I love you.

Garcia: (after scrambling around for a pen to write down Prince William's phone number only to lose access to the CIA server) Sleep well, sweet Prince.
JJ: Good night, crazy person!

Elle: (regarding Gideon's comment to Hawks) You said you'd send a postcard of a beach, why?
Reid: Yeah, you hate the beach.

Gideon: George Orwell said: "In a time of universal deceit - telling the truth is a revolutionary act."

Bruno Hawks: Twenty million dollars from Hassan will go a very long way to helping occupy my mind on a beach somewhere.
Gideon: The only beach you'll see is on a postcard I send you from my vacation.

JJ: (seeing the two agents off) Alright, we'll see you guys later, alright? We should do this again some time, okay? (sits down) Jerks.
Garcia: Come on, come on, quick! Quick, I know you're in here!
JJ: What are you doing?
Garcia: I'm looking for Prince William's phone number. The CIA is bound to have it.

Continuity error:

When Bruno Hawks asks Gideon if they caught their unsub (Mark Gregory, whose photo and preliminary evidence were still tacked up on the BAU's conference room board), Gideon replies that they did, but after Gregory had drowned eight women. In the episode involving Gregory (Charm and Harm), he actually committed six drownings --- the four initial ones followed by the one in Georgia and the one in South Carolina. The last two (the ones in Tennessee) were attempted but not actual drownings, as the first victim was saved by the motel clerk and the second was saved when Gregory was shot dead by Morgan.

Garcia: (whispering to JJ as the agent logs in to a CIA server on Garcia's computer) You do know what this means?
JJ: What?
Garcia: We'll be able to find out whether Princess Diana's death was really an accident.
Jennifer: I think that's exactly the kind of thing that got you on the list.

Princess Diana died from injuries sustained in a car accident on August 31, 1997, in the Pont de l'Alma road tunnel in Paris, France. Her car was being pursued by French photographers and there are a number of conspiracy theories that say she was assassinated.

"Charm and Harm"

Bruno Hawks: (referring to the picture of Mark Gregory) You catch him?
Gideon: Only after he drowned eight women.
Bruno Hawks: Sick bastard.
Gideon: He was.