Having this same issue. Noticed Photoshop CC 2019 was lagging today. Then discovered the GPU was not recognized. I've done everything from uninstall/reinstall all video drivers (tried different versions) as well as uninstalling and reinstalling Photoshop.
I have literally spent 6 hours troubleshooting and nothing has fixed it. Extremely frustrating.
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Several hours later:
Discovered the issue was a window update.
Uninstalled the update (then installed a utility to keep that update from updating automatically) and then the GPU would show up in PS, but I couldn't load or save files. Uninstalled PS. Reinstalled. Same issue. GPU would show up, but couldn't load/save. Turned off GPU in photoshop. Exited. Opened PS... and then it worked. Checked preferences, and GPU was there and turned on. So not sure what exactly fixed it... but it was the windows update. I create a RESTORE point called "photoshop works".