quilt is used to apply patches, but the series file is dynamically built by http://sources.debian.net/src/gcc-6/6.2.0-11/debian/rules.patch, which is included by the normal debian/rules file (see http://sources.debian.net/src/gcc-6/6.2.0-11/debian/README.source). Also, the patches include the diff against the gcc 6.2.0.