there's an indepth discussion on another thread somewhere... Basically the GBA has no real 3d hardware (mode7 doesn't really count), and neither does the GP. So 'cos they both have a similar processor you can compare (roughly) what they can do. The GBA has a 16mhz arm7 the GP has a arm9 which will go up to 133mhz.
Although MIPS comparisons are suspect, MIPS wise the GP comes out about Pentium 75-P90. So I suspect it could do 3D equivelent to PC games of that era
Incidently, porting that engine wouldn't be that difficult. I suspect there's a lot of ASM involved, but as both the GP and the GBA run on arm, the assembler would be the same.