Here's a concrete example, using gmpy2 [1], that exercises the code path you're proposing to remove. I had to cheat a bit, since the gmpy2 types *don't* (currently) buy in to the numbers ABC tower (though there's no good reason that they couldn't do so), so I registered them manually.

>>> import fractions, numbers
>>> import gmpy2
>>> x = gmpy2.mpq(2, 3)
>>> numbers.Rational.register(type(x))
<class 'mpq'>
>>> numbers.Integral.register(type(x.denominator))
<class 'mpz'>
>>> fractions.Fraction(x, x)  # code path exercised here ...
Fraction(1, 1)

Note that the numerator and the denominator of the resulting `Fraction` object are still of type `mpz`.

It _would_ be possible to remove the check and always use `math.gcd`, but it would result in a behaviour change: for the above code, we'd get a `Fraction` whose numerator and denominator were both of actual type `int` instead of `mpz`. Whether that's a desirable behaviour change or not is another question ...

[1] https://gmpy2.readthedocs.io/en/latest/