gcc6-aux attempts to fetch gcc-6-20170802.tar.xz, but after manually checking several mirrors, it looks like even the oldest GCC6 snapshot available on any mirror I checked is 20171206. So right now the port is unable fetch a distfile.
A commit references this bug: Author: ericbsd Date: Tue May 29 14:32:46 UTC 2018 New revision: 471093 URL: https://svnweb.freebsd.org/changeset/ports/471093 Log: Update lang/gcc6-aux disfile with missing changes PR: 227914, 228570 Approved by: wg mentor Changes: head/lang/gcc6-aux/distinfo
see https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=228570