Asking who or what Goldberry is, is a bit like asking who or what Tom Bombadil is, they're both enigmas and there's no certain answer to either question. So really, we don't know
She's not human, but she is magical in her own way. Readers have speculated that she's a lost elf, a Maia (a powerful Demi-God like being, like Sauron or Gandalf), or one of the Valar who are the Gods of Middle Earth. But none of those theories are strong enough to have become widely accepted.
She's described as being the daughter of the River Woman in the Adventures of Tom Bombadil, so she could also be a sort of water spirit. Tolkien said she represents seasonal changes in the Riverlands, so it's also possible that she's a different sort of magical being to those we see in LotR.
Tolkien ultimately said in one of his letters not to over-think Tom Bombadil, and by extension Goldberry. They are both a part of Middle Earth, but an unexplainable one
There's a longer discussion on what sort of creature she is here
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