So, in German, sex and gender are interchangable? Because they mean different things in English.
It would be more accurate to say we don't have a separate word for the latter.
I think I've seen "das Gender" used somewhere.
If you mean "sex" as in the sexual act, that is "der Geschlechtsverkehr" (lit. gender traffic). But that word is not really used except in documentaries etc. You would just say "der Sex". And sex as in gender is called "das Geschlecht".
I more meant "sex" as in the physical, biological factor and "gender" as a social construct.