It only takes into account the base armor of the gauntlets; which means that having heavy armor skill at 30 or at 100 (and thus changing the armor of the gauntlets) doesnt change the unarmed damage increase?

yep, fists of steel only takes base damage ito account, so a lvl 1 char with fists of steel and deadric gauntlets will do the same damage as a lvl 81 char would...stupid I know