A lot of people probably don't realize it but in the Nvidia Control Panel you have to re-add the Witcher 3 EXE manually if you are using the DX12 version otherwise it points to the DX11 EXE ... So EVERYONE needs to re-add the game manually:
Click the Add button
Then click the Browse button
The go to the Witcher 3/bin/x64_dx12/ folder and point it to the witcher3.exe file
Once you add it in the Control Panel the GeForce Experience app should be able to see it
If you are using the DX11 version you probably don't have to do this but I would just to be safe