If you have Xcode installed, gcc should be available. Check where it is with:
kamil$ which gcc
/usr/bin/gcc
Then make a user-land symbolic link from gcc-4.2 to plain gcc:
kamil$ sudo ln -s ~/bin/gcc /usr/bin/gcc-4.2
(Ensure the user-land bind folder is in your path via export PATH=...:$HOME/bin in your .bash_profile or .zshrc.)
Gem installed fine afterwards.