Can I use FEZ Cobra schematic as reference when designing USB client and host circuit for the G120?
Is the in-line resistors(33ohm) needed?
The required resistors are already included on the G120 module. You can optionally add 22pF and protection TVS to make your design more immune to noise and static discharge.
Great!
How about the USB_CON? Is it needed for the client port like in Panda II schematic?
Where did you see USB_CON?
I did just wonder if a USB_CON signal is needed like in panda design? or should client D+ be connected straight to USB connector?
you only need G120, power and few connectors 
Very simple, compare to EMX reference schematics.
Hello. I have my own 3.3V power module and I want connect G120 to USB. So I must connect D+ and D- to G120’s pins and USB Ground to ground pin. Is it right? Can I let +5V USB unconnected?
I know. I have 3.3V on another module to power G120.
Yes, that was my question. Because in schema of USB Client SP module is UD_VBUS line to gadgeteer port. And some resistors. … thanks.