Few days back, we provided instructions on how to get custom CWM recovery on the LG Optimus G Pro and we have the instructions on how to root the AT&T version of Optimus G Pro. Here are the steps.
1. Download and install USB drivers from here.
2. Download the root kit from here and extract it on your computer.
3. Connect your G Pro using any micro USB cable to your computer.
4. Go to Settings->General->PC connection->USB connection type and change to "Charge only".
5. Execute currentroot.bat on your computer and follow the instructions on the screen. You will now be told to change to MTP mode. Find the settings in Settings->General->PC connection->USB connection type.
6. That is it. Confirm the root status of your G Pro by running SuperSU.