well, it might be easier to just attach gccxml to the ITK formula and use the version distributed by ITK http://itk.org/files/gccxml/gccxml-2012-08-08.tar.bz2 (samtools is a good example of how to do this). That being said, if you are interested in adding gccxml back into homebrew, it would be good to ask them to add some new tags if they are stable enough.