Man, I forgot that PD Zero was delayed for so long that it didn't even hit the original Xbox. That game was on the "coming soon" screenshot section of my original Gamecube's retail box.
I've been replaying both GE and PD quite a bit thanks to this thread. And I'm glad that I bothered to unlock the extra difficulty modes for both games back in the day, as it lets me replay missions with all of the high-difficulty objectives but with lower enemy damage. Great for casual replays for someone who can't be bothered to relearn optimal routes 20 years later.