Playing devil's advocate, limiting Linux at first to GCC only allowed it to heavily use GCC extensions (inline assembly, statement expressions, and many others), which wouldn't be the case if it had to be portable to different compilers. The other compilers which could build Linux (ICC, sparse, now clang - compiling the Linux kernel with compilers other than GCC is not a new thing) had to implement many of these useful GCC extensions (for instance, clang is compatible with GCC 4.2.1), which made these compilers better.