I have a i7-4770 desktop with Z87 motherboard, 16gb RAM, and a AMD FirePro V4900 workstation graphics card. Since upgrading to LR 6, when I selected GPU acceleration in the Preferences/Performance tab I would get the "Graphics processor acceleration has been disabled due to errors" message. This continued thru LR 6.1.1. I didn't try it with LR 6.2 or 6.2.1 this past week as our focus was on the problems associated with those versions. Yesterday (14 Oct) I installed the latest FirePro driver 14.502.1045 after I rolled back LR to 6.1.1. With this new driver, I am now able to select the "Use Graphics Processor" GPU acceleration without getting the error message. There is some noticeable speed increase in the develop module. I can't quantify this increase but it seems some tools are slightly faster than the past several months using LR 6.
Lightroom requires up-to-date and supported Graphic Card to enable the GPU Acceleration.
Lightroom 6 had issues with some particular versions of AMD card as well as mentioned in below articles.