The Marines are utterly blindsided by the appearance of a monstrously strong pirate in East Blue. No one knows who he is, no one knows where he came from… except for Garp. Who thinks he knows and is hilariously wrong.
Now, the only reason Garp hasn’t hunted Riot down to turn him into a drumskin is because Riot’s first bounty poster had his “full” name on it. Garp gives the picture of “Portgas D Riot” the hairy eyeball, does some quick math, and is like “Roger you sly sea dog. Hahaha! Good trick!” I mean, Garp doesn’t stop chasing Riot because “the little shit stole my grandsons! I don’t care if he’s my grandson too! He’s getting a Fist of Love!” but. Garp’s not serious about it. And when the Melody does eventually catch up with the Anarchy, Riot reacts to Garp exactly the same way Ace does only with more fire. So it’s the Monkey D version of happy family time plus some really hammy acting when Riot “escapes” with the boys and his crew after the party’s over.