mkvmerge GUI version: 4.9.1
mkvinfo GUI version: 4.9.1
MKVExtractGUI-2 version: 2.2.2.5
While MKVToolnix is available for Mac, to my knowledge there is no GUI wrapper for the "mkvextract" function; this might be the only reason to consider using the Windows version. Not included in the Windows version installer, MKVExtractGUI-2 can be downloaded here.