dariush,
I have used these modules in the past and they are very inconsistens and do not work reliably. I ended up first making custom antennas and then finally rolling my own with a PCB based antenna which worked out to about 8cm very reliably with occasional reads > 10cm. Metal in the read field and other GP8 nearby read fields will interfere. I just threw out about 50 GP8s!
For the custom antennas, I followed the suggestions in their manual (2 sheets as I recall !) as far as impedance.
When I rolled my own I folowed the advice of the chip manufacturer EM Microelectronic (EM4095) and their datasheets.
Good Luck,
Peter House