Based on these numbers, than he should be about a 50 power rating.
Brett Gardner's average for last 3 years is 13.33 (17, 16, 7 HRs in last 3 seasons respectively)
54 power is a little inflated, but its less vs lefties and thats where you will probably not hit as many.
Him hitting 20 in a year is within the realm of possibility and sometimes thats just the way it goes.