kat and gunblade

when using ult with kat equipped by gunblade i was ignored by the high hp voli. while not being damaged i noticed something, i wasnt healing. not even a little. while i looked at numbers i was only getting passive self regen, none of the several hundred points of procced damage on voli was healing me. to test this i hit q and got a noticeable increase. when spinning i got more because i had a good amount of ap and ad by then. then i ulted on three people and no increase at all. why doesnt gunblade recognize ult damage as damage for its proc?