I would say you need at least 8 years on the team, and to retire with the team to be eligible for the rafters, The other question is does KG go to the Hall as a Celtic or Timberwolf?
KG had a long run in Minnesota and accumulated most of his career numbers there so there is a case to be made that he is more of a Minnesota guy, but the fact that he won a ring with Boston and really embraced the team as a leader makes him a Celtic through and through. My concern is if he is traded to another team it puts a dent in his Celtic resume I respect KG too much to say why, and this may be the difference between whether you retire the number or not. So for me playing his career out in Boston and retiring as a Celtic should put his number in the rafters.
Once he does that then it becomes his call what 'colors he wears' in Cooperstown. Might be a tough choice for him, I dont know.