Gender is an individual's perception of (among other things) their sex. Sex is which chromosomes they have.
For most mammals, that's XX for females, XY for males, or any of the (rare) aneuplodic sex chromosomal abnormalities like Kleinfelter syndrome (XXY, e.g. male calico cats), or (rarely viable) chimeric individuals where two embryos fused in the womb. For some mammals (a few bat & rat species), most arachnids, and many insects that's XX for female and just a lone X for male, and any aneuplodic abnormalities of the sex chromosomes that aren't fatal result in an abnormal female. For birds, most reptiles, some insects, some fish, some crustaceans, and some plants, that's ZW for female & ZZ for male, with similar complications to the XY system.
Sex is pretty simple. The vast majority of the time for humans, it's either XX or XY.
Which (primary and secondary) sex organs someone has is more complicated, because that can be altered. But it's still pretty simple, if not always what one would expect from the chromosomal sex.
Gender is complicated, because it's entirely social. It's not entirely clear which animals even have gender.
Sexual attraction is also complicated. The factors which determine it aren't well understood.
For most people indirectly, but yes. Whether you feel "male" or "female" is a core aspect of gender, and "being male" means having XY chromosomes, while "being female" means having XX. Physical sex organs & hormone production also tend to play into gender but aren't necessarily as fundamental: women don't stop being female after they go through menopause or have a hysterectomy. But females can feel that they should have been born male (and likewise the reverse), and can undergo hormone replacement, gender reassignment surgery, and act to comply with the societal norms for men. They'd still be female, but they'd be men. Man & woman are genders, male & female are sexes.
Of course for the vast majority of people their gender matches their sex. And it matches the sex hormones they produce, their sex organs, etc. We don't directly perceive our chromosomes, but we do perceive their effects, and those effects usually align with our gender.
For most mammals, that's XX for females, XY for males, or any of the (rare) aneuplodic sex chromosomal abnormalities like Kleinfelter syndrome (XXY, e.g. male calico cats), or (rarely viable) chimeric individuals where two embryos fused in the womb. For some mammals (a few bat & rat species), most arachnids, and many insects that's XX for female and just a lone X for male, and any aneuplodic abnormalities of the sex chromosomes that aren't fatal result in an abnormal female. For birds, most reptiles, some insects, some fish, some crustaceans, and some plants, that's ZW for female & ZZ for male, with similar complications to the XY system.
Sex is pretty simple. The vast majority of the time for humans, it's either XX or XY.
Which (primary and secondary) sex organs someone has is more complicated, because that can be altered. But it's still pretty simple, if not always what one would expect from the chromosomal sex.
Gender is complicated, because it's entirely social. It's not entirely clear which animals even have gender.
Sexual attraction is also complicated. The factors which determine it aren't well understood.