I have to agree with the OP. In my mind, Duncan will always be a center.
I realize in his early years he played almost stricty PF, next to Robinson, but even then his style of play was still more that of a center. After Robinson retired, his style of play has been stictly that of a center, even if he did play some time next to other legitimate centers.
In regards to KG, he is bar none the best defensive PF EVER (could also be considered one of the best defensive players, regardless of position). On offense, I would say his selflessness actually ended up hindering him. He could have put up much better numbers had he been more agressive on offense.
Ultimately, his legacy is downgraded due to the fact he played on a lot of crappy teams. Those teams in Minny would have been the worst in the league without him, ala Cleveland this year without Lebron. Just making the playoffs with so little talent surrounding him was an accomplishment.
I know it's impossible to say what WOULD have happened, but, swap Duncan and KG, and they probably have completely different legacies.