Power gain of an amplifier having input gain of 20W and what is the output gain of 20mW?

The power gain of an amplifier having an input of 20W and an output of 20mW is 0.001. Expressed in decibels, that is a gain of -30db. (log2 0.001 * 3)

If you meant an output power of 20MW (mega instead of milli), the gain is 1,000,000, or +60db.