I often hear or read “gender” where “sex” is the appropriate noun.

Problem:
“Gender” and “sex” are not synonyms.

Explanation:
The noun “gender” is a grammatical term that traditionally has been used to refer to categories of pronouns and nouns (masculine, feminine, and neuter).

The noun “sex” refers to the division of living things into the male and female state. Given that “sex” has become overloaded to refer to the act, it seems that a prudishness has developed about using a perfectly good noun — “sex” — to refer to the division of living things.

Sorry, but a person doesn’t have a gender; a person has a sex. Ditto for cats and dogs and other living things.

Solution:
Use “sex” when referring to people and other living things. Reserve the use of “gender” to refer to the kind of a pronoun or noun.