The Kickstarter OS X version is a simple zip file. First, it was available as the goodies part of Obduction then it was moved to its own "Obduction: Kickstarter OS X" title.

I tried to download it from both places but ended up with the same result:
The zip file is shown as being 8700 MB. GOG Downloader shows it as 9.32 GB. The downloads work but the file that is downloaded can't be opened. Verifying the file reports that it is damaged.

The Galaxy Client shows me the title in the Mac section but then tells me "Not Compatible" and doesn't allow me to download the file.

Personally, I find it a pita that Glaxy doesn't even allow me to download files that aren't for my chosen platform but I guess that is GOGs decision though it makes me even less likely to use the Galaxy client than I am Today. In any case, this is not about the client but about the fact that the zip file offered by GOG for the Mac version of Obduction is damaged.

As this file is almost 10 GB in size, people who have data caps won't be very happy to download it just to find that the zip file is damaged.

Actually, they are accessible in the same manner that you can do a manual install on the game page from the Library. Next to the "Install" button is the "More" button, and within the "More" button is "Backups & goodies". It doesn't matter whether it's compatible, you can download everything.

Or maybe you're experiencing Galaxy not showing your other games at all? When I hover the mouse cursor over the Library item at the top of the window, a menu appears that has the option to show all my games. Also, there are the standard search options. Notice the System option by default has selected the computer's system, and it can be changed.

Ahhh....yes...thank you. I missed that :)

I also managed to finally unzip the downloaded file by trying different unzip utilities but the created application is not running properly getting an error that has been reported ad nauseum already in all the forums about the missing .bin.

I wonder if the .zip was created on macOS 10.12 and the error is actually Apples less than stellar compatibility because I see that some people manage to play ok.

Hmm, does that mean you've read this other thread? In that case there were two .bin files plus the installer file. Did you get them all?

The actual problem with the missing .bin on the Mac is, as reported by Cyan in their knowledge base (http://support.cyan.com/kb_article.php?ref=1221-UPFV-9088) that Obduction is Metal based.

I went through oodles of backer messages, dev blogs, forum messages and nowhere did Cyan announce that they are using Metal for Obduction on the Mac. That technology didn't even exist when we backed the game...ok, I accept that technologies do change in 3 years but they did manage to have Obduction be a DirectX12 game but still support DirectX11...they could have, at the very least, announced that they were using Metal when they decided to do so.

But then, the backer forum at Cyan has a few messages complaining about the lack of communication between Cyan and their backers.

edited to add the link to the Cyan knowledge base

I've tried multiple unzipping tools, but only "Archive Utility" was able to unzip this file.