Is the main Atlantis Base Stargate CG because I think it looks beautiful. I'm not really sure about the shield in front of it though, but the way the thing lights up and the blue activating on the Chevrons, I was really impressed. I also liked the space one from the end of "Rising".
The Atlantis gateroom Stargate is a physical prop. Unlike the SG-1 gates, it was made mostly of rubber, and designed to easily swing down into hidden panels on the floor so the gateroom can be redressed as other parts of the city (it's much lighter and easier to move than the main SGC gate, because it doesn't have a big spinning part running through it). It does have a more metallic finish, and the material is slightly translucent, which allows some interesting effects with the "digital" symbols (until just recently when this thread prompted me to watch some episodes in HD and do some research on the
EMG fan group's findings, I'd thought the glyphs were front-projected when dialing out, because I could see them on top of the chevrons, but couldn't when the 'gate was idle or incoming. Turns out, they're always visible, because the points of the chevrons are actually translucent, and my cable connection and DVDs were just too low-res. Still not sure how they switched the symbols on the actual gate, if there was a custom programmable light-array behind each panel (what I'm leaning towards now) or what).
They didn't build an off-world Stargate for Atlantis,* so those are always CG which is why it's almost always just buried in the ground on a planet and not mounted on some kind dais or platform like on SG-1 or Universe. The Spacegate in the pilot was good, but it seemed that as the series used different VFX studios for different episodes, each studio made their own set of models for everything, so the CG city, Puddle Jumper, and Stargates all tend to change back and forth episode to episode later in the series, and sometimes scene to scene. I think the versions from the pilot are generally the best, with the exception of the second version of the city,
which is a little more "muscular," and is more detailed,
including little bits of echos of the details on the Stargate (which I like, because there's a weird artifact of the way the show developed that Goa'uld environments tend to echo the Stargate a lot more than Ancient ones, aside from these little bits of that version of the city).
And then there's the infamous spate of
eight-chevron Spacegates that started popping up mid-series, which some fans insisted was intentional and not just an error. It was an error.
*They eventually made partial pieces of the gate for Stargate for close-ups off-world, or so I've heard. I don't remember spotting anything like that with one exception you'll know when you get to, but I saw the props in the auction catalog, though it could just be from that one special case. Those off-world gates were either made from molds from the SG-1 gate, or redresses of one of the traveling gates. There are actually a lot more subtle differences than I thought between the SG-1 and Atlantis gates back when I made
my CG Stargates out of publicity photos and SD screencaps from VHS-taped episodes, so once you know what to look for, there are a lot of "tells" between the Atlantis gate and an SG-1 gate disguised as an Atlantis gate.