Same here, I managed to stop the update and play, but one DLC is bugged so I tried to verify/repair, and gog tried to update the game again, only to fail and display NOT AUTHORIZED, Help PLZ!

Also having an issue. Downloaded something, no information stating what the download is, 1.4gb worth.
Completed downloaded on left side of GOG interface, yet right side of display showing Witcher 3, stated 72% complete and just hung there.

Pressed pause-all on download, it stopped showing 72% on right side of screen.
Setup/Uninstalled (32 bit) shows in task manager along with
The Witcher 3: Wild Hunt - Game of the year both process in the 2-3% CPU range.
Then the GOG client closed along with all processes.

Reopened GoG, The right hand side, the play button is greyed out, above that it is showing allocating disk (0%)
On the left hand side download screen it's showing 0.0mb/1.gb (0.0mb/s)
Setup/Uninstalled (32 bit) shows in task manager along with
The Witcher 3: Wild Hunt - Game of the year both process in the 01% CPU range.
The performance monitor in Windows task manager shows the disk is very active 56%

>>>>>>

After roughly 12 minutes all process ended, disk use dropped and the Play button appears green and ready to go.

>>>>>>

Feedback
Website GoG design comment:
Would appreciate more information on what is being downloaded, a link to the download notes would be helpful

Hi.

Exactly the same problem here, only that I can't cancel the Download (Update 1,4 GB) and the PLAY - button is greyed out... unable to play my game. :(

yup, having the same problem, 1.4GB update, not downloading. Play Button Greyed out. grrrrr only got the game yesterday!! Have lip synch issue and now this!

Yup... and there seems no way to work around it. Sad, so sad.... :(

Two days later.... still no solution, no helpful replies. Very sad.