You said that Lion isn't a gem, but why do you think that? I guess you could say that he doesn't have a visible gem, but a lot of corrupted gems have their gemstones in their core instead of somewhere visible.
No one said it. The show has shown it multiple time. He can’t be a shapeshifted Gem or else we would see his gemstone, and even if he were a corrupted Gem, nine out of ten times their gems are not completely hidden. On top of that, corrupted Gems don’t take on the form of actual animals, just things that resemble actual animals. He also has shown a need to eat, sleep, and hunt for his own food. Tie all of that together with the fact that a regular lion with most of Lion’s features was seen with Rose in Buddy’s Book, and it’s pretty clear that Lion isn’t a Gem.