How to use the gcc installed in macports?

You can control the symlink in /opt/local/bin/gcc by using port select. You can see available version using port select --list gcc. Anything listed with mp- as prefix refers to MacPorts’ own port, gcc42 and llvm-gcc42 refer to the compilers shipped with Xcode by Apple.

Example from my system:

$ port select --list gcc
Available versions for gcc:
    gcc42
    llvm-gcc42
    mp-gcc45
    none (active)
$ sudo port select --set gcc mp-gcc45
Password:
Selecting 'mp-gcc45' for 'gcc' succeeded. 'mp-gcc45' is now active.

After that, either open a new terminal window or issue hash -r to make bash recognize the change.