The oldest version of GCC was compiled using another C compiler, since there were others when it was written. The very first C compiler ever (ca. 1973, IIRC) was implemented either in PDP-11 assembly, or in the B programming language which preceded it, but in any case the B compiler was written in assembly.
Similarly, the first ever C++ compiler (CPre/Cfront, 1979-1983) were probably first implemented in C, then rewritten in C++.
When you compile GCC or any other self-hosting compiler, the full order of building is:
- Build new version of GCC with existing C compiler
- re-build new version of GCC with the one you just built
- (optional) repeat step 2 for verification purposes.
This process is called bootstrapping. It tests the compiler’s capability of compiling itself and makes sure that the resulting compiler is built with all the optimizations that it itself implements.
EDIT: Drew Dormann, in the comments, points to Bjarne Stroustrup’s account of the earliest implementation of C++. It was implemented in C++ but translated by what Stroustrup calls a “preprocessor” from C++ to C; not a full compiler by his definition, but still C++ was bootstrapped in C.