That's computer science alongside with Church/Turing. Maths could have tried to claim it but they doubled down on formalism so they don't deserve it.
That said though incompleteness follows from nothing but logical implication itself so it's more fundamental than physics (try to imagine a physics without cause and effect that doesn't get you cancelled because Boltzmann) and philosophy (find me a philosopher who wasn't asleep during their logic lectures).
Well, it depends on your definition of truth and it could be the absolute truth by definition. A theorem is absolutely true in the same way that "a bachelor is an unmarried man" is categorically true.
The test to know if anything is an absolute truth is if it is called an absolute truth. If it is called an absolute truth, then it isn't an absolute truth. If it isn't called an absolute truth, then it isn't an absolute truth. Absolute truths don't exist. If someone tells you something is an absolute truth, stop listening to them.