blacklist: gfortran is now part of gcc

Library/Homebrew/blacklist.rb

@@ -75,5 +75,9 @@ def blacklisted? name
     The creator of Osmium requests that it not be packaged and that people
     use the GitHub master branch instead.
     EOS
+  when 'gfortran' then <<-EOS.undent
+    GNU Fortran is now provided as part of GCC, and can be installed with:
+      brew install gcc
+    EOS
   end
 end