How will Gadwick remember the actual amount of X mana spent?
Under current rules, the card would not work as intended with the current wording.

As the card Venarian Gold shows, it is, in principle, possible to refer to a self-chosen value of X for an enters-the-battlefield ability. However, as you might tell, it's quite an awkward wording and, if it got a functional reprint, would probably be handled more elegantly with a replacement effect ("As Venarian Gold enters the battlefield, [..]").

In any case, Gadwick, the Wizened's wording is different, makes no reference to "X as you cast [it]", and would not work under current rules. 107.3 and its subsections do not provide a definition of the value of a self-chosen X on an object outside the stack, because no currently released card would need it.

We have to expect a rules update for Gadwick, and, if they even remember the card, a change in Venarian Gold's oracle text.

Update from Eli Shiffrin, the Magic rules manager:


  Now Gadwick, the Wizened gets a new 107.3k rule to clarify that its triggered ability looks at the value X that comes from the spell it was, not the permanent it is.