From the Metal Gear Wiki

Gregg Berger (born December 10, 1950) is a St. Louis native known for his roles in both TV and video games. He has worked in many cartoons and video games, provided the voices for Jecht from Final Fantasy X, Grimlock in Robots in Disguise, Eeyore in Kingdom Hearts II, and various characters in G.I. Joe cartoons.

In Metal Gear Solid 3: Snake Eater, he provided the voice for The Pain.