Someone turning from male to futa is probably gender bender. If someone is born a futa and stays that way, that's probably not gender bender, at least by itself. Female to futa is only gender bender if futa is treated as a separate gender. Just because a girl has a dick doesn't mean they're not a girl. Though some stories create new genders in their settings, like abo.
Though when it comes to tags, or language in general, it really depends on how people use it rather than there being some sort of objective definition.