Uninstall OSX-gcc-installer without first installing Xcode
From what I'm seeing in the developer's scripts here, it looks like he is recommending that you install Xcode over the top of his "osx-gcc-installer" to be able to uninstall...At that point, you can run the uninstall-devtools script...

From the developer:


  If something doesn't work as expected, feel free to install Xcode over this installation.
  
  Once installed, you can remove Xcode completely with the following:

sudo /Developer/Library/uninstall-devtools -mode=all



Alternately, you can copy and paste the actual script content into a blank text document (maybe named "uninstall-devtools"), make it editable (chmod 755 uninstall-devtools), and then execute it (sudo ./uninstall-devtools -mode=all):

#!/usr/bin/perl
####################################################################################################
#
# Copyright (c) 2002-2011 Apple, Inc.
# Xcode 4.2
#
# NAME
#     uninstall-devtools -- Meta-script for running the various devtools uninstaller scripts.
#
# SYNOPSIS
#     sudo /Developer/Library/uninstall-devtools --mode=all
#     sudo /Developer/Library/uninstall-devtools --mode=xcodedir
#     sudo /Developer/Library/uninstall-devtools --mode=unixdev
#     sudo /Developer/Library/uninstall-devtools --mode=systemsupport
#
# Where the specified 'mode' value invokes the following devtools uninstaller scripts:
#
#     all:
#         /Library/Developer/Shared/uninstall-devtools
#         /Library/Developer/4.2/uninstall-devtools
#         /Developer/Library/uninstall-developer-folder
#
#     xcodedir:
#         /Developer/Library/uninstall-developer-folder
#
#     unixdev:
#         /Library/Developer/Shared/uninstall-devtools
#
#     systemsupport:
#         /Library/Developer/Shared/uninstall-devtools
#         /Library/Developer/4.2/uninstall-devtools
#
# The default value for 'mode' is 'all'.
#
# DESCRIPTION
#     This command runs the appropriate devtools uninstaller scripts according to the usage
#     specified on the command line.
####################################################################################################

my $do_nothing     = 0;
my $verbose        = 0;
my $warning        = 0;
my $debug          = 0;
my $help           = 0;
my $mode           = '';

get_options(
    'do-nothing' =&gt; \$do_nothing,
    'verbose' =&gt; \$verbose,
    'warning' =&gt; \$warning,
    'debug' =&gt; \$debug,
    'help' =&gt; \$help,
    'mode' =&gt; \$mode,
);

####################################################################################################

if ($help == 1) {
    print("Usage: $0 --mode=&lt;all|xcodedir|unixdev|systemsupport&gt;\n");
print &lt;&lt;"END";
This is a meta-script which invokes one or more of the devtools
uninstaller scripts, depending on which mode you select.

The recognized modes are:
all:
    /Library/Developer/Shared/uninstall-devtools
    /Library/Developer/4.2/uninstall-devtools
    /Developer/Library/uninstall-developer-folder

xcodedir:
    /Developer/Library/uninstall-developer-folder

unixdev:
    /Library/Developer/Shared/uninstall-devtools

systemsupport:
    /Library/Developer/Shared/uninstall-devtools
    /Library/Developer/4.2/uninstall-devtools

The default value for 'mode' is 'all'.
END
    exit(0);
}

####################################################################################################
# Determine if we are authorized to uninstall the devtools packages.
####################################################################################################

$| = 1;
if (($do_nothing == 0) &amp;&amp; ($&lt; != 0)) {
    die("ERROR: Must be run with root permissions -- prefix command with 'sudo'.\n");
}

####################################################################################################

my $uninstaller_script = $0;
my ($uninstaller_dir,$uninstaller_script_basename) = parse_name($uninstaller_script);
if ($uninstaller_dir eq '.') {
    die("ERROR: Must change to another directory before running this script, since the current directory is about to be deleted.\n");
}
my ($developer_dir,$developer_dir_basename) = parse_name($uninstaller_dir);

####################################################################################################

my @flags = ();
if ($do_nothing == 1) {
    push(@flags,'--do-nothing');
}
if ($verbose == 1) {
    push(@flags,'--verbose');
}
if ($warning == 1) {
    push(@flags,'--warning');
}
if ($debug == 1) {
    push(@flags,'--debug');
}

if (($mode eq '') || ($mode eq 'all')) {
    run_uninstaller_script("/Library/Developer/4.2/uninstall-devtools",\@flags);
    run_uninstaller_script("/Library/Developer/Shared/uninstall-devtools",\@flags);
    run_uninstaller_script("$developer_dir/Library/uninstall-developer-folder",\@flags);
} elsif ($mode eq 'xcodedir') {
    run_uninstaller_script("$developer_dir/Library/uninstall-developer-folder",\@flags);
} elsif ($mode eq 'unixdev') {
    run_uninstaller_script("/Library/Developer/Shared/uninstall-devtools",\@flags);
} elsif ($mode eq 'systemsupport') {
    run_uninstaller_script("/Library/Developer/4.2/uninstall-devtools",\@flags);
    run_uninstaller_script("/Library/Developer/Shared/uninstall-devtools",\@flags);
} else {
    die("Usage: $0 --mode=&lt;all|xcodedir|shared|systemsupport&gt;\n");
}
print("IMPORTANT: If you are going to install a previous version of the Developer Tools, be sure to restart the machine after installing.\n");

####################################################################################################

sub get_options {
    while (@_) {
    my $option_name = shift(@_);
    my $option_pointer = shift(@_);

    foreach my $arg (@ARGV) {
        if ($arg =~ /^--$option_name/) {
        my ($arg_name,$arg_value) = split(/=/,$arg);
                $arg_value = 1 if (!$arg_value);
        $$option_pointer = $arg_value;
        }
    }
    }
}

####################################################################################################

sub parse_name {
   my $name = shift;
   my ($dir_name,$base_name) = ($name =~ m{^(.*/)?(.*)}s);
   $dir_name =~ s|(.*)/$|$1|s;
   return ($dir_name,$base_name);
}

####################################################################################################

sub run_uninstaller_script {
    my $script = shift;
    my $flagsref = shift;

    if (-x $script) {
        my @args = ();
        push(@args,$script);
        foreach my $flag (@$flagsref) {
            push(@args,$flag);
        }

        system({$args[0]} @args);
    }
}

####################################################################################################