Well, that makes it a little easier.
Maxon:
AMax 32, 15W, 6V, P/N:236643
Speed constant 996rpm/V = 2988rpm @ 3V, no load.
Max torque (continous) 36.5mNm
Stall torque 127mNm
Planterary gearhead GP32A, 33:1, P/N:166163
Efficiency: 75%
Max continous torque 2.25Nm
Max intermittent torque 3.4Nm
This combination would give you a no load speed of around 90rpm and max continous torque of around of 0.9Nm.
At 0.5Nm torque output the motor pulls around 2.1A. (0.5Nm/33/0.75/0.00958)
It's armature resistance is 0.454ohms so you'll get a voltage drop of almost 1V across the armature.
That leaves you with about 2V for the actual "motor" which makes it spin at 2*996/33=60.36rpm.
The stall torque of the motor is, as stated, 127mNm giving you a stall torque at the gearhead output of roughly 3Nm which is slightly over the limit so you may want to current limit the drive electronics.
Lacking a motor with a 3V winding I think the above is a pretty damn good match.
It's going to cost you north of $300 though (single quantities).
And to get it out of the way:
No, I don't work for Maxon or any other motor manufacturer or distributor or retailer or anything like that ;-)