To enable GD library on your server, you will need to recompile your PHP with GD. You can recompile Apache and PHP from WHM -> Update Apache or you can also do it via SSH. To compile via SSH, you can use /scripts/easyapache.
If you have no experience in recompiling Apache and PHP, I suggest you to hire someone as if it fails then all your sites will stop working.
Note: If you want us to recompile your Apache contact us at support@webhostingdiscussion.netKailash