Motor power rating is electrical input power, not mechanical output power. Efficiency of cheap, poor DC motors can be as low as something around 50%. Efficiency of the cheap gearbox is also likely low, especially for large reduction ratios (using multiple stages of gears). Worm gears aren't very efficient, there's a lot of friction. The combined efficiency of motor+gear is likely around 20-30%.
Stall current is defined by the total DC resistance of the motor windings and brushes, and is of little use here. When operated near the stall current rating, the efficiency is likely very poor; you can think about it as the motor iron saturating and being unable to produce more torque, so larger part of that current is converted to heat.
Nominal current is something you need to know, this is the on-load current the motor is supposed to work against for the given duty cycle. Then you could apply P = U*I, but this is still electrical power, and you need to estimate the efficiency to know the mechanical output power.
So I suggest you take a look at motors rated for at least 10W.
This being said, if your duty cycles are short, most motors can produce 2-3 times the nominal torque for a short time (tens of seconds) without overheating.
The best way to know is to buy a motor and try it.