We do not have an accessory or mount to use a smartphone with the GP-PRO. Although you can use our smartphone gimbals with both a smartphone and most action cams, you cannot use our action cam gimbals with a smartphone.
With this answer often comes the question, "why go with the SS vs the SHIFT or SP-PRO Gen. 2?". The deciding factor will come down to which camera you plan on using more frequently. The only downfall of using an action cam with our smartphone gimbals is there are certain angles in which one of the axis arms is picked up in the footage because the camera does sit so far out. We have found the best way to avoid this from happening will be to hold the gimbal as you would hold a flash light, parallel to the ground.
If your action cam will be the primary use, you may want to go with the SS vs one of our smartphone gimbals.