gender gear in game should be tradeable for the other yes. Mogstation? no, they want you to buy one or the other (or both). Also stuff at the calamity salvager is tied to achievements, and since there is no achievement for buying gear from the mogstation the game does not "know" that you have it, and warns you against discarding it. they would need to create a system to track such things and currently this does not exist.

not to say that it won't in future but anything mogstation i don't think you'll ever see an equivalent gender gear swap for free.