Gnd pins at the power pin strip VS Gnd pin at the IO pin strip
In short, all the UNO ground pins are the same.

They are just there to make your life easier. At the end of the day, GND is GND. There is no technichal difference. Just note that is is sometimes better if you share the 3 GND ports. Overloading a port might damage it. Check the datasheet for current limitations of the pins.



EDIT

The pins (literal pins of the IC) themselves are also different on the ATmega IC.