I still get a little grin thinking about how weirdly simple this is: Monkey D. Garp doesn't have a canonical bounty listed in 'One Piece'. There's no official wanted poster for him in the manga or anime, and databooks never put a formal number on his head. That's because bounties in 'One Piece' are a tool the World Government uses to mark criminals — mostly pirates and rogue elements — as threats to the public order. Garp is (or was) a decorated Marine vice-admiral and a public face of the Navy, so the whole concept of the government putting a price on its own legendary hero's head is basically backwards by their rules.
That said, fans have long debated “what if” scenarios and misread some side material, which sparks rumors about a changed bounty. The only real way his bounty would ever change is if his legal status changed: if the World Government publicly branded him a criminal, defected to piracy, or committed an act of treason, then they'd issue or raise a bounty. In-universe examples of changes like that are characters who shift roles (becoming wanted after committing crimes or after being revealed as a member of a forbidden organization). For Garp, none of that happened canonically — he’s respected for fighting pirates like Gol D. Roger even if he had complicated personal ties to people who became enemies. Personally, I love that ambiguity; it makes Garp feel like the kind of legend who exists above bounties, more of a living myth than a wanted poster.