gnat-7/7.3.0-26 wrongly assumes that /usr/bin/gcc links to gcc-7. By chance, this currently holds in unstable, and gcc-7 is hopefully able to build a fixed version of itself.
gnat-8/8.1.0-11 wrongly assumes that /usr/bin/gcc links to gcc-8. It is broken in unstable, at least with default options. So the fresh gcc-8/8.1.0-12 fails to build with gcc-8/8.1.0-11. I suggest that gcc-8/8.1.0-13 Build-Depends: gcc-7 again until 8.1.0-13 is available on all architectures, then gcc-8/8.1.0-14 Build-Depends: gcc-8 (>= 8.1.0-13) Another option is to append --GCC=gcc-8 --GNATLINK=gnatlink-8 etc to each /usr/bin/gnatmake invokation. A second upload would also be necessary once the fixed gnat-8 is available, in order to remove the options, because such work-arounds have repeatedly resulted in broken crosss-builds or bootstraps.