The game has never had a Garnet, no, aside from some test driving here and there. I don't believe anyone's asked the mods about her appability specifically, but based on the wording in the FAQ, there doesn't seem to be a problem with apping her: "Characters who are possessed or share a body with another soul or sentient being can be tricky to apply for. By default, a player will always have the ability to control both." It seems to be saying that there isn't a problem with apping a character comprised of multiple characters (and the rest of the section is more specifically about whether multiple players could play different aspects of the character), but I could pop a Garnet-specific question to the FAQ to be sure!