If you are including novels, then yes, Garak is a 'good' guy. He clearly has a lot to atone for, and his struggle to put aside his worse nature for the greater good is probably one of my favorite story-lines from the novels. He's shown himself as a strong ally of the Federation, and considered the federation president one of his close friends.
...but even then
, he would never choose the Federation over Cardassia. He will go down with the ship.
Its worth noting that even in 'A Gutted World', Garak siding with the federation was not him turning against Cardassia. At that point, Garak realized that most of the command structure of Cardassia had been replaced with changelings. Or he suspected they had been, so the only way to help Cardassia was to side with the federation.
The closest thing you get to a Garak choosing the federation over Cardassia is in the Millennium trilogy, when everyone gets bumped into the future. But in that instance