Dear Sir or Madam,
I would like to dock my UR5 robot to a gearbox with a pneumatic test adapter with external thread. When the robot docks with the test adapter, it pulls the entire gearbox up with it. This must not happen. I want the robot to loosen up in its Z-axis before it docks, so that it can yield and not pull the entire gearbox up with it. I hope I could describe my problem to you well and you can help me further.
Here is a Video of my Problem
First off check the payload. are you updating the payload once the tool is engaged with the gearbox?
It looks like your tool has a little bit of draw during its stroke - How much does the tool pull the part in axially?
IMO, I would hold the gearbox down rigidly and active freedrive while engaging the tool. But if you have various gearboxes with various hole locations, the fixturing could get messy.
Check out the force mode program node.
You can achieve this fairly simple by using the Force wizard command. Insert the Force command, change type to Frame and modify the Z value to 0 N and keep the 150 mm/s (or test various speeds). Then you must have the commands to engage with the gearbox within the Force command.
Maybe it would be beneficial for you to add a bit of negative force in Z, to make sure the robot moves down towards the gearbox.
Since you have the Robotiq F/T sensor installed, you should be able to get a much more precise force readings than the CB’s built-in.
As mentioned already, it is also possible to freedrive with a script function (see the script manual for more information), which will allow the robot to move freely, when it engages with the gearbox.
In both cases, you should make sure your payload is setup correct in the Installation tab. 
looking at the video, the test probe looks out of square. if this is correct, then square it up and center it then your problem might go away.