72% always. If you adjust to having 120% most of the time, then you get smacked with a GW for 50% because the mage tapped you when you were below 40%, you're going to have to GTFO really quick.
Most who suffer from GW at all are ones who are typically a bit squishy to begin with the exception of Nasus and Mundo who sustain at all times. Swain is super squishy, Voli doesn't proc his passive until he's below the Nomicon threshold for GW, Aatrox is squishy as heck, Olaf's entire kit revolves around him being around the threshold that GW is at, Vlad is easy to burst down to 40% just hard to kill after bursting him because of his enormous amount of sustain, and then there's Fiddle.... He's super squishy and only his drain sustains him which is easy to stop with CC regardless.
The new GW is pretty much a buff and it's even more-so a buff since before you'd grab ignite to be able to have some kind of kill potential on these monstrosities and their disgustingly high amount of sustain, but now you're only reducing their sustain by 40% compared to 50% with ignite, though you can grab an Executioner's Calling I guess and waste a few hundred gold if you're a bruiser since you won't want the finished item given you're going to become a damage sponge later on.