Since the package is either not available, or using a different name, the first step is to find which is the case. Use

yum list available |grep gcc
yum list available |grep devtoolset

to find whether you have misspelled the package name, or if gcc is available under a different name. Check using yum info for the package. Its description should show the relevant version information.

According to Red Hat's article What gcc versions are available in Red Hat Enterprise Linux?, gcc 4.8 is available for Red Hat Enterprise 7. If you need gcc 4.8 for RHEL 6, you may have to compile it yourself.

Further reading:

• Installing GCC 4.8.2 on Red Hat Enterprise linux 6.5
• How to Install gcc 4.7.x/4.8.x on CentOS
• install latest gcc on rhel 6 x86_64

You will have to do it the old fashioned way. Get the GCC sources, build it by hand.

Any reason you are averse to doing this? (I can think of a few, but unless you do too, I don't want to add anything that might be confusing).

If you are hesitant to build from sources, I can probably provide some steps.

NOTE: The reason for that is the devtools with 4.9 is currently only available to Red Hat users with a valid license. Wait for a while, and eventually I am sure 4.9 devtools will be made available to CentOS users too like 4.8 is currently.

I am using CentOS 7.9 and I encountered the same problem after following instructions here to install and run gcc 11. I tried launching different versions of gcc and found only devtoolset-9 works, which corresponds to the file devtoolset-9 in /etc/scl/conf/ folder. So I copied devtoolset-9 to devtoolset-11 in the same folder, and gcc 11 gets working.

Install it by:

sudo yum install centos-release-scl
sudo yum install devtoolset-4

The first command installs and enables Software Collections Repository on your CentOS machine. That repository provides the devtoolset package.

While Devtoolset-3 and 4 are located on the Centos mirror site, yum will not be able to find them as they were not included in the repodata on the site. These were probably excluded because they are EOL (end-of-life), and have dependency bugs (Bug 1410152). Try using the older Devtoolset-3 packages here, which do not have the dependency issues:

https://copr.fedorainfracloud.org/coprs/rhscl/devtoolset-3/

If you really want the packages from the centos.org site, I went with wget to download all the RPMs, then recreated a local yum repo. However, you'll see the dependency issues and yum install devtoolset-3 will fail.

$ wget -r -p -e robots=off --directory-prefix="/mnt/local-devtoolset3-repo" --recursive --no-clobber --no-parent http://mirror.centos.org/centos/6/sclo/x86_64/rh/devtoolset-3/
$ sudo yum install createrepo
$ cd /mnt/local-devtoolset3-repo
$ createrepo --database .

Figured it out. cmake was using an old version of gcc/g++, not the one that my command line was picking up when I ran gcc or g++ myself. So, I had to export CC and CXX to the appropriate paths before running cmake.

export CC=/opt/rh/devtoolset-7/root/usr/bin/gcc
export CXX=/opt/rh/devtoolset-7/root/usr/bin/g++
cmake3 -G "Unix Makefiles" -DCMAKE_BUILD_TYPE=Release ..

Previously, I was only running the last line above.

To install the full tools-set including gfortran on centos 7:

yum install centos-release-scl
yum install devtoolset-8
scl enable devtoolset-8 -- bash

enable the tools:

source /opt/rh/devtoolset-8/enable 

you may wish to put the command above in .bash_profile

ref: https://unix.stackexchange.com/questions/477360/centos-7-gcc-8-installation

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

Now devtools-8 is available and it's possible to use it by following commands:

yum install centos-release-scl
yum install devtoolset-8-gcc devtoolset-8-gcc-c++
scl enable devtoolset-8 -- bash

It's explained in a little more detail in this answer on StackOverflow: How to install GCC/G++ 8 on CentOS. And of course, if you need GFortran, you can install it in the same way - the package is devtoolset-8-gcc-gfortran.

Probably you have a mess with env paths. Check files .bashrc or .bash_profile and remove old version path.

Last chance is to use direct/explicitly:

 source /opt/rh/devtoolset-7/enable