You are purchasing a gimbal mounting kit for the Cortex X8. 
It,s a 3D printed ready to use Gimbalholder with two 12mm rail-tubes ( distance 140 mm )
for mounting Gremsy Mini Damping Bracket ( T3 & S1 ).  The distance from cortex bottom plate is 40 mm. The length of each tube is 60 mm, enough for shifting the gimbal away from COG if necessary. The used material is more rigid than ABS and its weight is 94-gram incl. mount screws