As Gnar, the Q angle is decided by your current position when it returns, meaning as long as you're walking to or from the max range point, it will always hit, and if you are in lane, its easy to catch. Its a nice poke.
However, when chasing as Gnar (Or when trying to attack someone running in the bushes) around corners such as the toplane bend, you often are running to the right or left of the max range point, meaning to catch the return you have to stop moving entirely, or in some cases turn around, making it far more worthwhile to just drop it.
Draven's Q on return do not head to your current location, they head, more or less, to where you are heading, meaning only the smallest break in movement to catch them. Not only that, but he gets a MS boost to compensate for it.
Gnar's MS can be rendered almost useless if he proc's it on a Q, as he wastes it waiting for the Q to return.
A possible solution would be to draven-ify it, meaning it aims in front of the direction you're currently moving, to make it easier to catch.
This is only a problem when aiming it about 30 degrees from the line you are running on, as most the time its easy to just walk straight into it. I have attached an amazing picture with (G) Gnar, (X) Target, Blue for his Q direction, Red for his movement direction and green for his preferred Q return