I don't like
Kaia. It sounds like a trendy sort of name.
Gaia is alright. I don't know why people would say GAY-uh, since
Maia is a fairly well-known name, and
Gaia is a name from Greek mythology. I think I prefer
Maia though (that spelling). I have a friend with that name. It's pronounced MY-ah. I've never heard MAY-ah before. I don't think either would ever be as popular as
Maia, but
Kaia seems like it could be used more.