Today, I was viewing the directory http://jog.id.distfiles.macports.org/macports/distfiles/
and found that:
gcc-7.3.0.tar.xz 60M 02-Feb-2018 21:46
while
gcc-8-20170604.tar.xz 57M 04-Sep-2017 10:27
That is, gcc8 is outdated with respect to gcc7. How?
GCC 7.3 is the second point release off the GCC 7 release branch. It was released upstream on January 25, 2018.
gcc-8-20170604.tar.xz
looks like a development snapshot from Subversion trunk. The first released GCC 8 version was 8.1, on May 2, 2018.
See these resources on GCC release history: