Just picked up my Gitzo from the courier. I read the manual and couldnt find answers about two parts of the tripod.

What is the purpose of that slideable female screw on the top plate?

What is that detachable hook under the top plate?

The screw on the top plate is to prevent unwanted head rotation. The hook under the top plate is to hang weight for more stability, or just hang your bag instead of setting it on the ground.

If you are talking about the off center set screw that you can tighten from the bottom, it is to lock the tripod head on to the plate after you have screwed it on to the main center stud firmly.

The hook is to hang weight from your tripod to make is more stable. You can use what ever is available, but a camera bag or backpack can work out well, and you will likely have that with you all ready. You may need a short cord or some hook device to make it easy to attach and remove the weight.

Be sure to check that the your plate is firmly attached to your tripod. Use the provided wrench to verify the clamping screw is very firmly tight. There have been cases where it is not well tightened when delivered.