Gibson is the definition of a player that is slightly above average at most things but not great at anything. Calling him a rim protector is being extremely generous (he had 0 blocks in the playoffs that you claim he was great in). He's a tough, physical player that plays hard, but is best suited as an energy guy off the bench. He's not a starting caliber big man.
You know how I listed half the league's point guards ahead of Jeff Teague the other day. Well here's the leagues power forwards that are better than Gibson.
Griffin
Love
Aldridge
Millsap
Ibaka
Morris
Young
Green
Favors
Randolph
Saric
Davis
Nowitski
Williams
So again, you would rather have two average NBA players in Teague and Gibson over say Kyle Lowry? I would rather have Kyle Lowry for 2 years - 30 million per over 2 years of Gibson and 3 years of Teague.