So last night my Gatekeeper apprentice player (really low rung in the heirarchy) told me he was interested in the Gloaming. No problem, I look through the books briefly, do a google search and come up with some contradictory information. Then in today's game he says "after we finish in Sharn, let's go to the Gloaming!"
So I thought I'd do a bit more looking for some info on it as the next adventure is going to be set there ;) I've got the entry in the 4th ed Eberron Campaign Guide, along with Faiths of Eberron. Is there any other sources of information on this place?
Page 174 of the 3.5 Eberron Campaign guide has a couple of paragraphs.
Not that I know of. The only thing mentioned about the Gloaming is that it's a manifest zone of Mabar, and the rest I believe should be left to the DM.
Thanks guys 
I can now go crazy and make the Gloaming my own (which to be honest is likely what I was going to do anyway. If there was any information that contradicted my plans there's a good chance I'd have just ignored it. But I like to know when I'm ignoring canon 
).
I'm probably retreading territory you've already covered in your reading, but I'll list it anyway just to be prudent and cover all bases.
Firstly for the 3.5e material:
Eberron Campaign Guide has material on the gloaming on pg 174 as was already mentioned.
Next is Faiths of Eberron, pg 98
Finally you have Players Guide to Eberron, pg 58-59 has some minor mentions of the Gloaming.
In 4e you have ECG pg 135 - 137.
Can't think of anything past that. BTW would you mind posting up what you're working on? I'm a fellow fan of the Children of Winter and hearing other DM/players idea on that would be greatly appreciated. Trying to flesh out my own ideas and am suffering from a bad case of creative block.