Homebrew cannot find gcc

I'm having trouble getting homebrew to work on Lion. For some reason, it is picking up clang rather than gcc.

==> Upgrading gnutls
...
checking for gcc...
/usr/bin/clang checking whether the C compiler works... no

'brew doctor' says:

We couldn't detect gcc 4.2.x. Some formulae require this compiler. NOTE: Versions of XCode newer than 4.2 don't include gcc 4.2.x.

NOTE: I have already installed command line tools from XCode, including gcc.

$ which gcc
/usr/bin/gcc
$ gcc -v
gcc version 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2336.9.00)

How can I force brew to use gcc?