As far as I know, you must click one by one. That's what I do, but I'd be thrilled to be proven wrong.
That's why there such clamour about GOG not flagging installer version changes.
Thank you for your answer.
That is what I want from GOG Downloader. I would like for it to check download dir and check for md5 or whatever it needs for updates. I'm not programmer so I don't know how hard it is to implement this.
I hope that someday they implement it and make GOG more perfect. :-)