I think that Gary did exactly what his character was written to do... It set the stage for trek to follow, I think that episode had much more depth and history to any others. It was a great one also.
Would he have made a great movie character, YES maybe even better than Kahn if done right( let the rants start haha). It's good they didn't bring back another "lost" TOS character though, same thing twice would have been lame.