All right so this is what i've (not) found - I could barely find any direct benchmark comparisons between pentium 3 and the 750CXe (Which Gekko is a 750CXe + increased registers and floating point performance) Initially I thought I had looked at a few game benchmarks of quake III arena but checking them again they were either running at different resolutions on each system (lower res on the pentium hence me thinking it outperformed) or used different graphics cards. I've found benchmarks of individual systems that are as close as possible to the Xbox and gc, but then I couldn't find the same benchmark for the other cpu for apples to apples comparison. Lots of dead links and data that makes no sense.
These are the most concrete comparisons (because they're the only direct comparisons I could even find that weren't just forum posts) i've found which point to the power pc beating pentium III by quite a bit but only show one or two benches so it's far from comprehensive -
http://www.infohq.com/Computer/appleG4-pentiumIII-showdown.htm
^ The above video makes sense given what i've read about the 750's superior floating point performance (which gamecube would have even more per clock), even if this is surely a best case scenario that apple cherry picked. And I understand FP doesn't tell the whole story. After all if it did i'd imagine the gamecube's fixed t&l pipeline would've been a non issue.
http://www.anandtech.com/show/858/13
In anandtech's 2001 comparison of the GC and Xbox consoles, they claim "In terms of raw performance, the Celeron 733 (4-way set associative L2) will outperform the PowerPC 750 running at 500MHz in any of the synthetic benchmarks we've seen. We can only assume that a 733MHz CPU with a 133MHz FSB and 8-way set associative L2 cache would only be faster than the Gekko giving the Xbox the CPU performance advantage." I couldn't find anything to prove this claim.
Besides floating point performance the other gekko advantage would be 4x the amount of general purpose registers the Xbox has, which to my understanding has a lot to do with memory efficiency. No surprise there looking at the rest of the gamecube's memory setup. Not to mention the faster front side bus of the gamecube (166 v. 133).
So yeah, again it seems like the power pc 750 commands a per clock advantage, though to what point (realistically) I can't tell. It's probably safe to say both gekko and the Xcpu had advantages given the type of task and they were overall pretty comparable.
If anyone has any more concrete comparisons or developer statements, that'd be appreciated. Besides factor 5's (apparently PR) statements, the only other dev I remember commenting on gekko were the guys behind gladius ; they were really impressed with the speed of the chip and how painless it was to get things running. Can't find the comment now though.