The G5 doesn't have full little-endian support and to my knowledge can _not_ boot standard ppc64le kernels (which are targeted at the POWER8+ ISA). The only big endian stuff I could find when I tried to put Linux on my G5 was 32-bit, which does run on the G5 but likely means it's Linux support dies with its older PPC Mac brethren.
Oh, pardon my mistake! I've always wanted a G5 but have neither the space nor electrical capacity for one.
They are definitely power hogs. And slow.. Core 2 era Intel machines run circles around them.
But their obscurity just makes them kind of neat!