The Gomu Gomu no Mi doesn't exists in old Devil Fruit books. The Nika Fruit did. The WG changed it from the Nika Fruit, to the Gomu Gomu No Mi, and therefore the Nika fruit does not exist in any of the modern Devil Fruit Books, the Gomu Gomu no Mi does.
As to why Luffy doesn't know the true name after eating it, is either a retcon, or will be explained down the line.