Most problems with security specs and libraries that implement them are communication problems. They involve people incompletely describing or understanding their requirements, capabilities, or threat model. Often this also involves providing/using interfaces that are not ergonomic (https://github.com/google/mundane/blob/master/DESIGN.md), which in turn can be caused by the spec trying to do too much (as XML Signature does).

I don't know how GPT could help with that. If anything I would expect it to bias toward things it has already seen, which is the opposite of what you want when writing a new spec/library aiming to avoid past mistakes.