No, it is not... quite. You can dive with super speed and gain an incredible amount of altitude. Let me reiterate: when a plane is in flight it can gain altitude by either adding power to the engines or using a dive to gain enough speed to then climb.The Gryphon's can, at best, maintain a level flight path for a short distance but then they slowly glide towards the ground. And, unless I've missed something, I have yet to have my Gryphon gain altitude on a long glide.Putting a Gryphon into a dive and then "pulling up" does not actually lead to a gain in altitude just an increase in speed.
At around 19 seconds in, the person on their griffon uses their super speed, or whatever it's called, to climb to a much higher altitude than it was currently gliding at, and then doing another dive to regain super speed, so that they could climb to a higher altitude again if they wanted/needed to, but in this instance they finished the course, so climbing again isn't necessary. Assuming that a person's reflexes are good enough, and there's plenty of space below you, I believe it should be possible to keep a griffon in the air indefinitely with the dive/super speed/climb mechanics.