You can use the centos-sclo-rh-testing repo to install GCC v7 without having to compile it forever, also enable V7 by default and let you switch between different versions if required.
sudo yum install -y yum-utils centos-release-scl;
sudo yum -y --enablerepo=centos-sclo-rh-testing install devtoolset-7-gcc;
echo "source /opt/rh/devtoolset-7/enable" | sudo tee -a /etc/profile;
source /opt/rh/devtoolset-7/enable;
gcc --version;