What is the purpose of metal layer on the bottom of a ceramic patch antenna for GPS
So what is the purpose of that metal layer ?


The metal layer is fundamental to the operation of a patch antenna. Patch antennas use a ground plane and that is what it is.


  Bottom of the ceramic patch is fixed to ground plane on the top of PCB
  through adhesive. So there is not contact of metal layer (which is
  there at the bottom of ceramic patch) to ground plane on PCB.


Read the data sheet for the patch antenna and follow the instructions contained in that data sheet. It's quite possible that the data sheet will tell you to either provide insulation to the PCB, not route tracks in that area or have exposed copper that can make a contact. This can only be determined by reading the data sheet.