It's Harry Potter.
Garry sounds absolutely ridiculous, because "Garry" is also a name. So when you say Garry Potter, it sounds like you're talking about some other "off-brand" wizard.
I know it's Russian pronunciation convention to convert the first sound to a G when a loanword begins with the "voiceless glottal fricative" (AKA the "h" sound), eg hormonal becomes "Gormonal".
But Garry Potter is simply absurd.