Federal GMM is what inspired me to design the OCW load development system. That ammo, and other factory recipes like it, prove to a large degree that while all rifles are not as different as most handloaders would have you believe. A rifle is not some mystery unto itself necessarily--although there will be subtle differences in barrels that will cause them to prefer their own favorite distance to lands with the bullet.
But put Federal GMM or one of Hornady's match ammo loads into you .308 and if it doesn't shoot great, there's something wrong with the rifle and/or shooter. That's a simple fact.
All that said, you can outperform Federal GMM with a good handload, it's not that hard to do. Just use good brass, the right charge of powder, and thesame bullet that Federal is using (either the 168 or 175 SMK).