I think 'gender' is an internal feeling tbh, so yes I guess you can decide your gender.
I do not think this means everyone else should have to play along that you are the opposite sex though, in areas where sex actually matters of course.
Kinda depends how you define gender too. To me the modern meaning of gender is kind of an internal essence, rather similar to a soul. To some 'gender' is just another word for 'sex' and this is where the confusion comes in. I mean, theres 50+ different genders and counting..so quite clearly doesnt mean sex or anything close to it
