Try:

yum group list

To see if yum can find the Development Tools under Available Groups. If it fails, try:

yum clean all
yum group list

And see if it can find it. If it still fails, you might not have the correct mirrors enabled to resolve it.

try the clean command again and then:

yum --disablerepo=\* --enablerepo=base,updates group list

If you can find it. Great! Install with:

yum --disablerepo=\* --enablerepo=base,updates groupinstall "Development Tools"

If not so, can you give us the output of:

yum repolist all

EDIT:

Thank you for the output.

I found an article which state the following: "To install the CentOS Development tools, you will need to be able to connect to your Virtual Server using SSH, and work as the root user."

Try switching to user "root" (if you don't know the root login credentials, you can try sudo su. sudo also might work but I see you already have tried that...) and running the above commands.

Lastly, you can try and enable these "sources" repos:

• CentOS-7 - Base Sources
• CentOS-7 - Extras Sources
• CentOS-7 - Updates Sources

And than running yum update followed by groupinstall.

Let me know if that doesn't work. Good luck!

An alternative of source /opt/rh/devtoolset-4/enable is

source scl_source enable devtoolset-4

The above shell script scl_source is more elegant than using a hard coded path (may be different on another machine). However scl_source does less because /opt/rh/devtoolset-4/enable uses scl_source and other stuff.

To use scl_source you may have to upgrade package scl-utils

yum update scl-utils  # old scl-utils versions miss scl_source

Quick copy-paste

echo 'source scl_source enable devtoolset-4' >> ~/.bashrc
    # Do not forget to change the version ↑

Source code for curious people

An example of scl_source source code:
https://gist.github.com/bkabrda/6435016

The scl_source installed on my Red Hat 7.1

#!/bin/bash

_scl_source_help="Usage: source scl_source <action> [<collection> ...]

Don't use this script outside of SCL scriptlets!

Options:
    -h, --help    display this help and exit"

if [ $# -eq 0 -o $1 = "-h" -o $1 = "--help" ]; then
    echo "$_scl_source_help"
    return 0
fi


if [ -z "$_recursion" ]; then
    _recursion="false"
fi
if [ -z "$_scl_scriptlet_name" ]; then
    # The only allowed action in the case of recursion is the same
    # as was the original
    _scl_scriptlet_name=$1
fi
shift 1

if [ -z "$_scl_dir" ]; then
    # No need to re-define the directory twice
    _scl_dir=/etc/scl/conf
    if [ ! -e $_scl_dir ]; then
        _scl_dir=/etc/scl/prefixes
    fi
fi

for arg in "$@"; do
    _scl_prefix_file=$_scl_dir/$arg
    _scl_prefix=`cat $_scl_prefix_file 2> /dev/null`
    if [ $? -ne 0 ]; then
        echo "Can't read $_scl_prefix_file, $arg is probably not installed."
        return 1
    fi

    # First check if the collection is already in the list
    # of collections to be enabled
    for scl in ${_scls[@]}; do
        if [ $arg == $scl ]; then
            continue 2
        fi
    done

    # Now check if the collection isn't already enabled
    /usr/bin/scl_enabled $arg > /dev/null 2> /dev/null
    if [ $? -ne 0 ]; then
        _scls+=($arg)
        _scl_prefixes+=($_scl_prefix)
    fi;
done

if [ $_recursion == "false" ]; then
    _i=0
    _recursion="true"
    while [ $_i -lt ${#_scls[@]} ]; do
        _scl_scriptlet_path="${_scl_prefixes[$_i]}/${_scls[$_i]}/${_scl_scriptlet_name}"
        source "$_scl_scriptlet_path"
        if [ $? -ne 0 ]; then
            echo "Can't source $_scl_scriptlet_name, skipping."
        else
            export X_SCLS="${_scls[$_i]} $X_SCLS"
        fi;
        _i=$(($_i+1))
    done
    _scls=()
    _scl_prefixes=()
    _scl_scriptlet_name=""
    _recursion="false"
fi

Package managers are great, but (understandably) won't generally include the latest version of software. CentOS being a distribution particularly focusing on servers, favors stability (and extensive testing) over having the latest and shiniest. Here are your solutions:

Change distro

Other distributions like Arch Linux, Sabayon Linux or Debian "Sid" include newest versions a lot faster. You may consider changing distros.

Build it yourself

If you're stuck with CentOS, your best bet is to build gcc from source. It means getting the source code, making sure the dependencies are already present on your system (if not, I'm sure you can get them from the CentOS package manager), compiling the source into a binary executable, and installing the executable on your system.

It's not the easiest way to do it for sure, but you'll learn a lot about Unix in the process. Make sure to read the document a few times to make sure you get everything, and ask questions if something specific is unclear.

Be patient

At some point, this version (or newer) of gcc/g++ is going to enter the package manager. If you wait (up to several months, I'm not very familiar with the CentOS release cycle), you'll ultimately get what you want.

I ran into the same issue that you are facing and this is how I got it fixed:

1. Just want to be careful, you need to exist the Terminal and open a fresh one to start. This way, you are not under any devtoolset's bash.
2. Go to /opt/rh folder, run command ls -la to see if you have any devtoolset-* folder there. Let's say you have devtoolset-8, proceed step 2.
3. Go to /etc/scl/prefixes folder, if you don't see devtoolset-8 file, you can create a new one as devtoolset-8, and type 1 line: /opt/rh, then save and quit that file.
4. Once you are done, you can call: scl enable devtoolset-8 -- bash w/o any error. Good luck