How does one define gender? Can we?

(Originally posted by myself here).

What defines our gender?
It is not:

• career (nurse, doctor, engineer, secretary, teacher, gardener, sheet metal worker)
• mannerisms (‘camp’, 'butch’)
• sexual orientation
• interests (fashion design, IT, sewing, cooking, cars)

What criteria would we then use to determine our gender?
If we think there are only two genders, our gender is the same as our 'sex’, hence it is easily 'explainable’ why there is no dependence on interests, career or appearance in defining our gender (e.g. a male fashion designer still identifying as male). (This is because of the assumption that sex is defined by anatomy or chromosomes and that there are only 'categories’ - male and female).
If we are accepting of the gender spectrum, the anatomy/chromosomes of the individual are irrelevant. Do then the interests/behaviours/appearance become the defining criteria? But using these depends on gender stereotypes e.g. if I am interested in car racing, IT or engineering, I must be male. Conversely, if I like dresses, talking with friends for long periods or gardening, I must be female. And what if I liked ALL the above? Male? Female? Both? Neither?!
How then do we define gender?
Does one need to be a stereotype to be that gender? Does one need dysphoria to be a different gender?
What in the blazes is the point of a gender marker anyway?!