yum install centos-release-scl-rh
yum install devtoolset-3-gcc devtoolset-3-gcc-c++
update-alternatives --install /usr/bin/gcc-4.9 gcc-4.9 /opt/rh/devtoolset-3/root/usr/bin/gcc 10
update-alternatives --install /usr/bin/g++-4.9 g++-4.9 /opt/rh/devtoolset-3/root/usr/bin/g++ 10

The official way to have gcc 4.8.2 on RHEL 6 is via installing Red Hat Developer Toolset (yum install devtoolset-2), and in order to have it you need to have one of the below subscriptions:

• Red Hat Enterprise Linux Developer Support, Professional
• Red Hat Enterprise Linux Developer Support, Enterprise
• Red Hat Enterprise Linux Developer Suite
• Red Hat Enterprise Linux Developer Workstation, Professional
• Red Hat Enterprise Linux Developer Workstation, Enterprise
• 30 day Self-Supported Red Hat Enterprise Linux Developer Workstation Evaluation
• 60 day Supported Red Hat Enterprise Linux Developer Workstation Evaluation
• 90 day Supported Red Hat Enterprise Linux Developer Workstation Evaluation
• 1-year Unsupported Partner Evaluation Red Hat Enterprise Linux
• 1-year Unsupported Red Hat Advanced Partner Subscription

You can check whether you have any of these subscriptions by running:

subscription-manager list --available

and

subscription-manager list --consumed.

If you don't have any of these subscriptions, you won't succeed in "yum install devtoolset-2". However, luckily cern provide a "back door" for their SLC6 which can also be used in RHEL 6. Run below three lines via root, and you should be able to have it:

wget -O /etc/yum.repos.d/slc6-devtoolset.repo http://linuxsoft.cern.ch/cern/devtoolset/slc6-devtoolset.repo

wget -O /etc/pki/rpm-gpg/RPM-GPG-KEY-cern http://ftp.scientificlinux.org/linux/scientific/5x/x86_64/RPM-GPG-KEYs/RPM-GPG-KEY-cern

yum install devtoolset-2

Once it's done completely, you should have the new development package in /opt/rh/devtoolset-2/root/.

The easiest method by far is to make use of a binary build that's provided through a YUM repository. One such option would be to use the hop5.in repository. Specifically this page: gcc - Various compilers (C, C++, Objective-C, Java, ...). They're providing 4.8.2 which should work with CentOS 6.3 or 6.4. You might want to do an update prior:

$ sudo yum update

The other option would be to make use of the Developer Toolset, specifically the bundled version provided by Scientific Linux.

• http://linux.web.cern.ch/linux/devtoolset/

Following the installation instructions you'll basically do the following 2 steps:

$ sudo wget -O /etc/yum.repos.d/slc6-devtoolset.repo \
    http://linuxsoft.cern.ch/cern/devtoolset/slc6-devtoolset.repo
$ wget -O /etc/yum.repos.d/slc5-devtoolset.repo \
    http://linuxsoft.cern.ch/cern/devtoolset/slc5-devtoolset.repo

$ sudo yum install devtoolset-2

Update #1

The hop5.in YUM repository appears to have been removed, so the only recourse is to make use of the devtoolset method highlighted above.

Additional examples for installing via devtoolset are highlighted in this GitHub Gist: Installing gcc 4.8 and Linuxbrew on CentOS 6.

I feel foolish because I figured it out right after posting, but I figured I'd share because it isn't intuitive and hopefully this will save someone some time.

The package you want to install is not gcc. You want to install gcc-toolset-9. You will notice after installation that you will still get 8.x if you do a gcc --version. gcc-toolset-9 installs to /opt. The idea is to give you a separate development environment. See this documentation. After installation you can run the updated gcc with /opt/rh/gcc-toolset-9/<username>/bin/gcc.

As @Stephen Kitt pointed out, you can actually get a shell with the updated toolset by running scl enable gcc-toolset-9 bash. Running this command will open a new bash session with the environment variables appropriately updated.

You can also run gcc v.9 directly with scl enable gcc-toolset-9 gcc <your_gcc_args>. See this documentation for details.