Since there isn't a clear, comprehensive and universally-accepted definition of what 'gender' actually is, using it in surveys, questionnaires, etc., is introducing a big blob of uncertainty into your conclusions.
Using 'gender' to the exclusion of 'sex' is like dismantling the foundations of the building you're constructing.
I'm OK with 'gender' meaning 'the social expectations of men and women', as the authors of the article say.
In this way, gender identity became enmeshed with the concept of ‘gender’ developed by feminist philosophers to describe the social expectations of men and women (Mikkola, 2024).
This is how I experienced whatever it was I experienced as a little girl - whether 'gender dysphoria' or 'gender questioning', I was certainly gender non-conforming, and wouldn't/couldn't accept the social expectations of being a girl who was going to grow up to be a woman. My 8-year-old self would have added a 😱emoji to the idea of having to be a woman.
But that is totally irrelevant to data collection, unless the research is specifically about gender-nonconforming girls. It has no place on a form or questionnaire.
It's not often that anyone on FWR refers to a Judith Butler book as an illustration, but relax, I'm not going to quote from it, I'm just going to show the cover, which is a photo of two sisters. The little girl on the left reminds me of me - awkward and unhappy about having to present as a girl.
I think that was to do with 'gender', I can't think now else to explain the disconnect between what I was told I was, and what I felt I was, or wanted to be.
Spoiler alert: I worked it out, and I wish all gender-nonconforming little girls could learn the same thing: my sex is female, but I can be anything I damn well want!
Nobody helped me work that out, but mercifully nobody hindered either, with puberty blockers or binders or talk of transitioning.