Como eles poderiam usar o Atlantis Stargate quando foi destruído em um episódio anterior?

No final do episódio seguinte, você deve ter perdido alguma conversa.

De 5x11, "A tribo perdida" :

Jackson: "I don't think I'll be around to find out, though."

McKay: "Yeah I heard they're, um, sending you back to Earth."

Jackson: "Yeah, I guess I need some major patching up and recovery, and Landry prefers I do that there instead of here, so..."

McKay: "Well, the Daedalus has already picked up a Gate, uh, leftover from the old Gate bridge, and uh, my team is installing it as we speak, so we should have you home in, uh, no time at all."

Jackson: "That's good, that is... good."

Está razoavelmente bem estabelecido que os Stargates são móveis / intercambiáveis.

Neste caso específico, uma das portas da ponte Carter-McKay foi usada para substituir a que foi destruída pelo dispositivo Attero. Esses portões se tornaram redundantes após a destruição da estação intermediária.

De o Stargate wikia

The Stargate was eventually replaced with one of the gates from the McKay/Carter Intergalactic Gate Bridge. The gate shield still operates with the new gate, suggesting that the shield generator for the Stargate shield is either not actually inside the Stargate itself or it is somehow an extension of Atlantis' primary shield generators. Also, because dialing another galaxy is also not dependent upon the actual gate, that function remains as well.