The .ebuild file passes a "--target" option to the econf command. Newer versions of the GMP configure script do not support this option. The fix is to simply delete this line from the .ebuild.
Created attachment 2741 [details, diff] patch for gmp-4.1-r1.ebuild
see Bug 5935 for a patch to the newer ebuild *** This bug has been marked as a duplicate of 5935 ***