First use yum search to find the correct package for your environment for our example we will use 5.6
yum search "php" | grep -i gd
The -i in grep just ignores the case of the search phrase, very handy in cases like this when it is written in capital on some of the docco and not others. Now that we can see a list of the packages available as shown below
php-ezc-EventLogDatabaseTiein.noarch : Contains the database writer backend for
php-gd.x86_64 : A module for PHP applications for using the gd graphics library
php54w-gd.x86_64 : A module for PHP applications for using the gd graphics
php55w-gd.x86_64 : A module for PHP applications for using the gd graphics
php56w-gd.x86_64 : A module for PHP applications for using the gd graphics
We can select the package we want and install it with yum
yum -y install php56w-gd.x86_64
After the install has finished you will need to restart your web server so that it loads the new ini files for GD, for our example it is Apache
/etc/init.d/httpd restart
All Done, you now have GD install have fun with those images
