I read this on the GPU FAQ:
When you launch Lightroom, it runs a test on the graphics card. If the test fails, the graphics card is disabled even if it meets the minimum requirements. Walk through the troubleshooting steps to try to resolve the error. Some graphics cards, despite meeting the minumum requirements, may never be compatible with Lightroom for graphics acceleration.
As Lightroom and Photoshop are different programs, despite the fact both are called "Photoshop", it's not a guarantee if it works for one that it will work for the other.
There are other threads about similar issues with similar cards.
Are you on LR 6.6? Did you have these issues with 6.5.1 also?
If not, you may want to roll back. Here's how: How do I roll back to Lightroom 2015.1.1 or Lightroom 6.1.1? | The Lightroom Queen
Simply replace 6.1.1 with 6.5.1.
I forgot .. if you are not using a high resolution display (i.e. 4k), there not a big speed gain with GPU acceleration turned on, depending on the power of your CPU.