Hit 'em baby, one more time.
The most important bit of Undead Fortitude is:
If damage reduces the zombie to 0 hit points
There is clearly a gate where the Zombie is at 0HP. At that point, they can make their save and possibly continue with 1 HP, but they have cleared the 0HP gate to initiate the GWM bonus action melee attack.
The zombie may finally end up at 1HP, but that doesn't change that they were reduced to 0HP and then made a save to finally end up at 1HP.
GWM only requires that you
reduce a creature to 0 hit points with one [melee attack].
And you have done this, it just didn't stick :)
The progression went from:
- Zombie at more than 0HP
- GWM hit succeeds, reducing zombie to 0HP.
- GWM allows for bonus action attack from reducing to 0hp.
- Zombie makes CON save and succeeds. HP now at 1.
- GWM can be used for bonus action attack to attack again because of Step 3.
Comparison with Avoiding Disintegrate by saving at 0HP to stay at 1HP.
The following errata answers what happens when a Barbarian with Relentless Endurance can avoid disintegration because they can avoid 0HP with a successful save.
Q: If the damage from disintegrate reduces a half-orc to 0 hit points, can Relentless Endurance prevent the orc from turning to ash?
A: If disintegrate reduces you to 0 hit points, you’re killed outright, as you turn to dust. If you’re a half-orc, Relentless Endurance can’t save you.
It's clear that the save won't save them, as they have still reached the 0HP gate that Disintegrate dictates.
GWM and Undead Fortitude fall under the same gate reaching of 0HP.
A way of looking at this is the Zombie is brought down with the hit. It's makes the save and rises again from the (un)dead. But at one point, the zombie was at 0HP and down.