Installing Sqlite3 Gem Error
It would appear that XCode 4.2 doesn't install gcc-4.2, but instead llvm-gcc-4.2. Clearly Apple feels that LLVM is mature enough to superseed GCC.
There are two options I can think of. You should be able to choose either and be OK.
Use this GCC installer.
Symlink LLVM using
sudo ln -s /usr/bin/llvm-gcc-4.2 /usr/bin/gcc-4.2.