"Gnosis--the Greek form of "knowing""

Aye. For the Gnostic, it's the only kind of evidence that one desires or seeks. This is why Gnostics deal in experience rather than faith and there is no Priestly intercessor: for the Gnostic seeks direct personal revelation.

Such an experience is not necessarily easily communicable to another (and it's certainly not 'provable') unless that other has shared an equivalent Gnosis.

Very much like falling in love, it's a powerful subjective experience. And one open to personal interpretation, depending on how the individual having such an experience chooses to understand it.