Is this gimbal ... brushless motor based or servo based?
Also, you must decide whether to mount the CC3D on the quad or on the gimbal.
If you mount it on the quad, it is treated as a true gimbal and the CC3D doesn't know the true angle of the gimbal since there is no sensor there. You will adjust some settings that depend on the exact number of degrees the gimbal travels for a particular change in gimbal control signal, but it will never be perfect, especially considering the servo type of gimbal when servo arm is non-linear when it has moved far away from center.
If you mount it on the gimbal, then CC3D is set up as if it were controlling a quad and can compensate for non-linearities of the round servo arm in the linkage and move the gimbal "perfectly". It might even be possible to get it to directly control a brushless motor gimbal, but that is outside my experience.
Finally, this could be a technically challenging setup. Do you have more time or more money?