I always understood ‘gender’ to be a grammatical term in the past. Using it this way is a newer thing. Sex was used to indicate male or female in the biological sense.
Most European languages assign gender to nouns. English used to have that feature but has gradually gotten rid of it. Until recent years, people would refer to ships and countries as female. For example if you were referring to the USS Constellation on a voyage, you would say she is sailing. Similarly, if you were to say that France was going to become an ally of Spain, you would say she is becoming an ally of Spain.