We should probably not count automatically created messages (like notices of receipt or the like) as meaningful responses. In a way to quantify support quality, the time to first meaningful response and time until some kind of solution is reached are probably the most useful times to look at.

This includes mail ordered bombs, btw.

Then I really wonder why in post number 14 the user is saying that there were no actual humanly written messages at this location? Maybe the message was not saved there or he overlooked it.

I did check the spam mailbox, and have not received anything from GOG Support. except the auto-reply one.
https://support.gog.com/hc/en-us/requests/37813

There are 5 messages there:
1. From me, open the ticket, April 05, 2017 09:52
2. From me, April 05, 2017 14:05
3. From me, April 05, 2017 14:09
4. Monday at 19:33. After 19 days, the issue is assigned to DrCat, and he/she replyed to me
5. From me, close the ticket, Monday at 20:47

Success!

I was able to solve the problem by following a simple suggestion from one of the GoG support technicians: try to install on the Desktop, then move the installed game to the Applications folder.

This worked. There seems to be a problem with the installer updating an existing installation, but it can be installed on the Desktop.

-dZ.