OK, this is the start of an answer and I'll add more if/when I find it. I'm going to mark it CW so that anyone who finds something additional can go ahead and add it.
Checking out the online compendium, we see two sources in Dungeon/Dragon magazine that look like they might be helpful. We also have several flavor text references to Gibbeth from warlock powers. I'll cover those in a moment. Let's take a look at the Dungeon Mag reference first. Issue 207 (can be found here starting on p3) has an adventure dealing with Gibbeth. Here's the first bit of lore:
Gibbeth is a green star with an unknowable being
at its core. This being carries an eternal curse that
will drive everyone who sees its true form mad when
the end times come. A scion of Gibbeth is this being’s
presence made manifest in the world. An elderly
eladrin named Ilshaeren is the voice of Gibbeth, and
he leads the Order of the Endless Night.
There are several more bits of useful information scattered throughout the adventure such as:
Ilshaeren is bent on learning Gibbeth’s true form.
He is wholly devoted to the menacing green star,
which (according to myth) reveals its true form
only at the end of the world.
The text also mentions that more can be found in Monster Manual 2. Something we should look into. The text is on page 197, but contains no real additional information from what I've already recorded here in the answer already.
