I have put a new gpc snapshot at:
http://www.math.uni.wroc.pl/~hebisch/gpc/gpc-20060215.tar.bz2
It consists mostly of bugfixes (many related to ISO compliance), but there also some new features. Main change is to argument handling. In particular, `const' discriminated strings are now passed by reference and there is new `const var' mode, which forces passing argument by reference.
For more detail see:
http://www.math.uni.wroc.pl/~hebisch/gpc/NEWS-20060215
and
http://www.math.uni.wroc.pl/~hebisch/gpc/Fixed-20060215