Though there are remains of scraps of clothing and a few pieces of (damaged) jewellery or other trinkets, nothing found particularly sticks out to you as being related to Gil.

Grok is well aware that though you could attempt to smoke out the tunnels, not knowing where they lead is the main problem. A well placed fireball could, depending on how the network is connected, do what you need; it could also burn up to the surface, if one of the passages leads upwards.

Spoiler: Star Chart
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The star chart is typically viewed as heresy by both Bahamut and Tiamat followers. It predicts - via astronomy - the invasion by Bahamut into Avernus, not to destroy Tiamat but to free her. This act will coalesce her five heads into one, and as a single Chromatic Queen she will join with Bahamut to lead all dragons into a grand new future just as they had worked together originally when they had first been born as gods.

The Church (as they like to be called) or Cult (as they are typically referred) of Prismatic Platinum is extremely small currently and lacks any notable followers or influence, but what they do have is a surprisingly robust ability to spread their propaganda - even if most people don't actually understand it, the pictures are nice to look at.



Spoiler: Skull
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The skull is of an Uridezu - a rat-like demon that is typically of minimal intelligence. They were often used as cannonfodder or other menial labour by more powerful demons, and rarely held any actual power. Because of this they are often more than willing to serve mortal masters who summon them to the Material Plane - even the worst mortal is often preferable to their treatment in Hell.

Craven and easily scared, they nevertheless held some degree of power and influence over the rats of the Material, and those demons that freed themselves and ran amok frequently used swarms of rats and dire rats as scouts and cannonfodder of their own. Uridezu were known to be able to summon other creatures of their kind, though rarely did so for fear of incurring a debt. Their bites are known to paralyze, and their tails could be used like whips to trip or disarm their foes.

Given its position among the wererats things, this skull may have been a small object of worship for them, or used as a sign of the leader's dominance and power.