I think we should use the binutils in %GNUARM%\bin, that is the cross-binutils' directory. Maybe those binutils are prefixed with some prefixes like gnu-arm-ld.exe. But I'm not sure that the tools of gnuarm.org are useful for a gba compiler... :?:

Maybe we can try to use binutils from http://www.devkit.tk/, or try to build it from scratch (that's not so difficult)