The gimp2 portfile includes this line:
compiler.blacklist-append {gcc-4.2 >= 5646}
This means that MacPorts will check if the program gcc-4.2 exists. If so, it will run "gcc -v" and examine its output. If the build number mentioned in the output is less than 5646 then gcc-4.2 will be blacklisted (i.e. not allowed to be used).
In your case, MacPorts apparently determined that gcc-4.2 does exist, but running it caused it to crash, because /usr/bin/i686-apple-darwin11-gcc-4.2.1 doesn't exist. This points to a broken Xcode command line tools installation.
What version of OS X and Xcode do you have? If Lion or later, please upgrade to Xcode 4.6, then visit the Xcode application's Preferences window, click Downloads, and ensure the command line tools are installed and up to date. Hopefully that's all that's needed. Xcode 4.3 and later aren't supposed to contain gcc-4.2 anymore, so if the problem persists and gcc-4.2 is still on your system after installing the latest Xcode correctly, gcc-4.2 will have to be removed manually.
