We would suggest the following, login into the server using SSH.
A forceful cPanel update is recommended,
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. Select 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.