* https://en.wikipedia.org/wiki/Buginese_language
It appears to be a cultural construct.
Gender in the sense of "the social roles and norms on top of biological sex" is indeed a construct, though heavily informed by the biology that they're based on. Biological sex is very much real and not a construct.
I am confident in this fact because I learned it in elementary school decades ago and it is impossible for humanity to discover new information that updates our world model. Every English speaker knows that “plasmas” and “Bose-Eisenstein condensates” are made up.
https://en.wikipedia.org/wiki/Intersex#Prevalence
https://en.wikipedia.org/wiki/Klinefelter_syndrome
https://en.wikipedia.org/wiki/XXYY_syndrome
https://en.wikipedia.org/wiki/XXXY_syndrome
https://en.wikipedia.org/wiki/XXXYY_syndrome
https://en.wikipedia.org/wiki/XXXXY_syndrome
https://en.wikipedia.org/wiki/Trisomy_X
I assume you will be one of the advocates for my nobel prize
edit: I'm sorry you specifically mentioned gametes, we can talk about diploids and haploids if you wish and how our bodies are such complicated machines that any sort of error that can occur in our growth is guaranteed to at scale