I am looking forward to migrate my c++ code to C++11, which might requires at least g++ 4.6. So what is the GCC installed on Mountain Lion?
Apple doesn't ship compilers with their App Store OS versions (Lion and now Mountain Lion) so the answer is none.
It does provide an llvm-gcc binary that serves as gcc as part of Xcode and the developer tools. This is a free download and you can use it to bootstrap to whatever version of gcc you please if the shipping version isn't to your liking.
Here is gcc version that ships with Xcode v4.3.3
Air:~ mike$ which gcc /usr/bin/gcc Air:~ mike$ gcc --version i686-apple-darwin11-llvm-gcc-4.2 (GCC) 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2336.9.00) Copyright (C) 2007 Free Software Foundation, Inc. This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.