I've just had the same problem, and looking for a solution that runs on Linux, led me to looking in the Memcardrex code, I've come up with the following:

dd if=<file.gme> bs=1 skip=3904 count=131072 of=<file.srm>

It turns out the GME files are really just the same as SRM files with a 3904 byte header in front, the above commands strips that header and leaves a raw memorycard file.