To install GD2 using WHM/CPanel, login into the server using SSH as root.

A forceful cPanel update is recommended,

/scripts/upcp --force

1. Enter this command /scripts/easyapache
2. Select option 1 "Previously Saved Config"
3. Select "Start customizing based on profile"
4. Select the next Apache version to upgrade or select Next
5. Select the version of PHP 4 or 5. Do not make any changes if you're not sure. Keep it as it as and select Next
6. Select the minor version of PHP.
7. Click Exhaustive Options List
8. PHP section has "GD" make sure you select it and select any other additional modules you require and select Next

This should start compiling Apache and PHP depending on the options selected. Check the phpinfo() page for a GD section.