A. A gym.

( Deb's T-Shirt in one early scene reads "Bishop's Body Shop" - so that might be the name of the gym.)

Q. Where is it supposed to be on the show?

A. Somewhere in the Miami.

Q. When did we see it on the show?

A. Several times in the second season.

Debra (Dexter's sister) is still traumatized by her close call (last season) with the Ice Truck Killer, so she starts spending an inordinate amount of time working out at the gym.

While there, she meets a guy named Gabriel, and has a brief affair with him (before deciding he's not the one for her and dumping him for Lundy).

We see the gym for the first time in Episode 2 (of Season 2), "Waiting to Exhale", when Debra finishes working out on a StairMaster, and then tries punching a bag. But she hurts her hand - which is how she meets Gabriel. He offers to tape her hands, but she has scary flashbacks of the Killer binding & gagging her with duct tape, so she leaves in a hurry.

We see the gym again (at night this time) in Episode 4, "See-Through". Deb is working out with a dumbbell and keeping an eye on Gabriel (who's on the lat machine). They make eye contact, exchange names, and the next moment we see them in the bedroom, as she's rushing him out of his clothes. Still paranoid, she insists on handcuffing him to the headboard before climbing on top (leading to an awkward moment when her brother accidentally walks in on them.)

The gym shows up again in Episode 7, "That Night, A Forest Grew ", where Deb & Gabriel are running side by side on treadmills.  He's talking, but she's tuning him out, and listening to soothing classical music on her headphones.

Q. What is it actually in real life?

A. A gym exterior and a studio set.

Q. Where can I find it in real life?

A. This is an odd location.

I was told that the gym didn't really exist, that it was just stock footage of the exterior, and that the interior was a series of sets (at first built in the studio, and later expanded to a larger set built in a Long Beach hotel lobby.)

But then I found out that the stock footage turned out to be of a local gym: the exterior shown on the series is actually the L.A. Fitness gym, at 201 S. Lake Ave, in Pasadena, CA.

That's at the southwest corner of Lake & Cordova Street, two blocks south of Colorado Blvd.  The side seen in the exterior shot is actually the north side, of the building, which faces Cordova. This is the gym exterior shown in Episode 2.

But to make it even more confusing, a completely different gym exterior  (which I still haven't found, and don't plan to look for) is shown in a night time shot in Episode 4. It looks somewhat similar to the original gym, but the street trees are completely different. And if you take a close look, so is the building. But when the gym exterior is shown again in Episode 7 (in the daytime), we're back to Pasadena again. (Looks like the producers dropped the ball on that one.)