Truly if Guts was only a tool to Griffith, then his opinion would hold no weight. This however is not the case.
Hmmm, I thought about this point too. And I'm torn, as I can't decide whether or not Griffith wanted Gatts to "belong" to him out of friendship, or mereely to keep himself sharp.
There's evidence for both sides of this argument, I believe. Griffith does seem to relate to Gatts more than to any other member of the Hawks. It's Gatts he sends on the special missions, he asks Gatts if he approves of Griffith's methods, etc. However, he DOES tell the princess that none of the Hawks are his friends.
I believe that Gatts is included in that statement and that Casca was right in saying that Gatts was Griffith's motivation to excel. He knew Gatts might be able to surpass him, so he needed to stay one step ahead.
Then again, maybe you're right... in the end, Griffith couldn't keep up with Gatts, which is evidenced in how easily he lost their second duel. Maybe he stopped seeing Gatts as a motivator and started seeing him as a friend? In any event, it's clear that he wasn't expecting Gatts either to leave or be better than him.
Thanks for the kind words on my first post!
~Shaughn