@Greg So Vegapunk stated that there is no Gum Gum Fruit in the "DF Encyclopedia", but there is this question from SBS volume 45:
"D: Hey, Odacchi! I got a question! About Devil Fruits!! If Kaku and Kalifa said they wouldn't know what their Devil Fruit were until they ate it, how come Shanks knew what Luffy's Gomu Gomu no Mi was before he ate it?? Tell me! Tell me now!! Pleeeease tell me!!! P.N. Shirogitsunekko
O: Well, I think I'll eventually get to explaining more about Devil Fruits in the main story, but you see, there's a book of Devil Fruits. It has info about all kinds of fruit names and abilities, but few of the fruit can actually be identified by their shape. In the case of the Gomu Gomu no Mi, there was a picture and everything, but for Kaku and Kalifa, they were talking about how they wouldn't know until they had finally eaten the fruit. That was in Volume 40."
This clearly implies that
a) a newer version of the book exists, probably edited over time and scrubbed of vital information or
b) that Oda just forgot and this is a small error on his part.
You think we will get some elaboration on this part?