the gist of it is "sex" is what you are from a reproductive-y, chromosome-y perspective and "gender" is the role you play socially - e.g. little XX humans don't pop out the womb as "female", that's the set of behaviors / expectations 'n' all that that get placed on you when society sees you have a vagina people are born as whatever sex they are and get assigned a gender associated with their bits, then later their "gender identity" (their own sense of what set of behaviors / expectations align with them) develops and either matches the gender assigned to them or not people that have a gender identity = assigned gender match are cisgender (although that word isn't well liked sometimes) people that have a gender identity =/= assigned gender mismatch are transgender I personally don't think that "gender reveal parties" matter one iota for reference but if you wanna be finicky then yeah "sex reveal parties" would be a better name - who gives a shit frankly