Here's why:
* Scientific name: The scientific name for a killer whale is *Orcinus orca*, and that doesn't start with "G".
* Common names: While "killer whale" is the most common name, other names include "orca" (which is also often used as the scientific name), "blackfish" (which is less common and sometimes refers to other species), and "grampus" (which actually refers to a different type of dolphin).
It's best to stick with the well-established names for killer whales: "killer whale" or "orca".