I don't think I've ever seen a mission where anything more than a reference to an Astral Gate is mentioned - if you could mention which you you saw yours at, that would be good to know.
As for general new things, all I can think of is Street Magic, which is supposed to be the go-to book for magic and astral things, though it focuses more on meatspace magic and spirits, without a great deal of expansion of the Astral aspect (I do hope that this is somewhat fixed in 5e); though it does briefly mention that physics might be different (being able to fly, some being entirely filled with water, etc). For the most part, I think that other planes are intended to be entirely up to the GM, so the lack of specific information is likely intentional.