The Hersh pretty much nailed it...
The ONLY consistency between hook sizes is within the same hook style/model number from the same manufacturer.
Case in point, below are two size 32 hooks. The smaller one is a Model 277 from Mustad (discontinued) inside of a size 32 Tiemco Model 518 (also discontinued):
Even a size 16 between hook styles/model numbers from the same manufacturer can differ widely because the size differences are relative to what they are being compared to within a single model number.
Since the flies I tie are typically representations of insects and the shank from the back of the eye to just before the bend is what I have to work with, I choose my hook sizes based on the length of the hook shank in millimeters and I don't worry about what size hook it is supposed to be.
In regards to hackle gauges; even though I have a few, they are relative to the gape of whatever hook the hackle gauge manufacturer used as a guide and what they think is the right size hackle for a particular hook. Even sized hackle like Whiting 100 packs is subjective to the gape of the hook you are using or if you are winding hackle on a bare hook or palmering hackle over a dubbed body.
For that reason, I find the best hackle gauge to be the hook I am using...
YMMV