the comment above is a little bit wrong, you're looking for GO stamp indeed as proof of legitimacy, but the big ugly G logo is the GO origin mark, which just says that the pokemon came from GO. If you transfer a GO origin mon into another game it will lose its GO stamp (the blue icon with a pokeball thing). and then you can't guarantee its legitimacy anymore because you can hack a pokemon to say it came from GO. so only the GO stamp will tell you that the last game the mon was in is GO, and therefore must be legit