There is another way to manually turn off the GPU.
Follow the below instructions :
- Quit Lightroom.
- Press windows and R keys to open run window and type appdata and hit enter
- Open Roaming> Adobe> Lightroom> Preferences
- Open Lightroom 6 Preferences.agprefs with Notepad
- Change the following line: useAutoBahn = true to the following useAutoBahn = false
- Save and close that file.
- Launch Lightroom and check.