I was in a similar situation, and thought that GP would serve two purpose (solid Ball Head and Gimbal for long lens). But some reviews I read (normal users like us) on the internet (dpreview, nikoncafe, fredmiranda, or some other site, do not remember it now), did not give good feedback on it's gimbal functionality.
Hence I finally went with Arca Swiss Z1 with regular screw knob. So, far I have had no issues with my Sigma 50-500 OS. I have not yet decided, if I should go for a side kick or a regular gimbal head.