My reasoning was:
Gil has access to everything "worthy" (and all NPs fall into this category) that the human species has obtained/created/used.
Gil doesn't have access to VS.
Thus, VS is not part of what the human species obtained.
If I understand it correctly, the reason that VS is not in Gil's treasury is that it was never used in the legends of the Nasuverse. As such, it wasn't recorded in human history in the sense it has to be to materialize in GoB.
The legends mention the existence of VS, but it was never tied to any "heroic" action, and as such, it's not a NP humanity obtained, even if the historical Karna did.
To add to that, Karna is a demi-god with high Divine Spirit aptitude. At his death, he sublimated into Surya. He was an outcast and the juicy parts of Mahabharata only happened because Karna was proud of his Divine descent. It's safe to say that he's not very close to humanity.
Formally, considering that the EX rank is reserved for things that are unquantifiable or just so insane that stacking pluses onto A doesn't really do the NP justice, if you have an EX rank slapped onto a thing which has one function (i.e. absolute destruction), it's safe to say that that NP does that one function real damn well.
Other than that, VS is praised for being able to slay Gods. As someone already mentioned, it's not Anti-Divine not because it has some particular affinity against divinity, it's simply strong enough that it can wear that tag on its shoulder. On top of that, it was the ultimate weapon of the King of (Indian) Gods, Indra, who was himself nervous about wielding it. Karna's background in CCC calls it the "greatest spear" at one point.
To be fair, we don't know what the extent of Excalibur's power is when all seals are removed. The fully released Excalibur which should be close in power to the sword that slayed Sefar is probably stronger in its output than anything else on the plane. It is likely above VS as such, but in its regular form, I don't see why VS wouldn't be more powerful.
As far as Ea goes, I understand it's contestable, but I'd give VS the advantage in a beam-clash, since VS is a more abrupt release of insane amounts of energy, while Ea has a more controlled, chanelled output of still insane, but not as insane amounts.