Two easiest methods:
#1 -Look at your groundspeed and figure it in NM/Minute. Take that and multiply it by the Ft/NM figure.
Example - 180 kt GS, Standard 200'/NM IFr climb gradient:
180 Kt GS = 3 NM/Minute * 200'/NM = 600 FPM
#2 - To find percentage (%) of climb gradient look at the 100 KT GS column, take the Ft/Minute figure and divide by 100.
Example - 100 KT GS, 400'/NM (because 200'/NM climb gradients are never listed, they're assumed):
100 Kt GS at 400'/NM is 666 FPM / 100 = 6.66%