Isn't GCC written in C++ now? I think they are probably aiming for a very old version of GCC written in C (or that's all they can really hope for).

The now-defunct Aboriginal Linux project was doing a similar sort of bootstrapping, and the build dependencies for GCC was a big issue:

http://landley.net/aboriginal/

To work around this, it never used anything later than gcc-4.2.1 from 2007, while we're now on GCC 8.2.

http://gcc.gnu.org/releases.html

EDIT: Yes, it appears GCC uses C++: https://lwn.net/Articles/542457/