My question is: Can I connect GND to 6v so the voltage difference of the led is 6v??? (12v-6v=6v)
question is...how much current to power the led is required?that circuit is overkill... and yes the same circuit would be fine, r2 is doing the work to protect the diode.
Where's the 6V coming from? If you have a 6V supply, why don't you use it directly instead of trying to subtract?