Gender of the voice as described in SSML voice
element <https://www.w3.org/TR/speech-synthesis11/#edef_voice>__.
Values:
SSML_VOICE_GENDER_UNSPECIFIED (0):
An unspecified gender, which means that the
client doesn't care which gender the selected
voice will have.
SSML_VOICE_GENDER_MALE (1):
A male voice.
SSML_VOICE_GENDER_FEMALE (2):
A female voice.
SSML_VOICE_GENDER_NEUTRAL (3):
A gender-neutral voice.
