How to do pocket in GSD.

You can make a pocket of a surface using the Split or Trim commands. You need to make an extrusion into the surface, fill in the end of the extrusion, then split or trim the original surface with the extrusion. Doing Trim saves a step, depending on what exactly the pocket should look like.

If you want to make a pocket of a solid, you can make another solid that is the shape of your pocket, then split the original solid with the new solid.

Let me know if you need any clarification or something.