A modern desktop PC is usually constructed in such way that the air flows from bottom and front, takes up the hot air from your components and blows it out in the back and top. That includes the GPU so if it is not faulty, there is nothing to change.

If you do not have a fully botched rig and the airflow is somewhat intact then keep your PC as it is and do not point additional outside fans at it. Especially not at the open side panel. Just make sure it does not stand on a carpet and has a few centimeters of space in all directions.

If needed replace your fans for better ones or add more. Just make sure that more air goes in than out to have some pressure from inside. Otherwise you will suck in more dust than you kick out.