There are a few Chaos legions not in the Eye of Terror.
Typhus for example seems to just wander the universes spreading disease, Huron Blackheart (hate him though I do) and his renegades wander the universe...wait, are we including renegades?
Problem is that the Space Marines outnumber Chaos by quite a margin.
I can't see why the Cadian Gate is such a worry. Doesn''t Abaddon have a Planet Killer and a number of Blackstone Fortresses. One less since the Eye of terror campaign, but that just means he won't have to worry about Eldrad any more. If all Chaos actually worked towards the same goals they would rip through The Cadian Gate...But they never do, and there are usually at least three different armies arranged against them. Imperial Guard, Space Marines and Eldar.
And Chaos are often alone. Dark eldar are never any help, Orcs fight everyone,. Necron have their own agenda, Tau usually fight with the Imperials when they have to choose between that and fighting them, Tyranids fight everyone ever and so are in Chaos' way more than anything else...Hell, even within chaos there are different plans, most the legions hate each other, Ahriman is obsessed with getting into the black library and thus is never in the battle field, Fabius Bile is too busy experimenting, Kharn will kill anyone and anything in his path....
The fact that Chaos can do anything is nothing short of a miracle when you consider that...Gah! Evil is never supposed to be outnumbered...