Here is why in my mind you're overrating Gortat:
1) his 15ppg number is inflated. Steve Nash makes everyone better offensively, maybe more than anyone in the league. He knows his guys and he knows where they need the ball, and where they'll be most effective. In truth, Gortat is a middling offensive center who is a good finisher but needs help getting to the hoop. Before Nash helped him hike up his stats, he was still getting 74% of his shots around the rim from assists. With Nash, and the corresponding bump, it went up to 84%.
He also had no good rebounders around him. Frye, Morris, Warrick, none of those guys are decent rebounders. With another good or great (2 of them on your team), and without Nash's help getting looks, and with a better second (or first, depending on how you see Curry) option in Gasol, I think his value to you goes way way down.
On your team, I think he's more of a 12 and 8 kind of center.