To those of you, like me, who were never able to install the patch either via direct download or via GoG Galaxy, I managed to work around the issue.
First of all, the minimum requirements for the installer in the GoG.com catalogue page have been updated to say it requires Mac OS X 10.9 and higher, when it used to be 10.8 or higher on initial release. This is not true for the game, but apparently only for the GoG installer.
Terrible Toybox specify that the game requires Mac OS X 10.7 or higher, and in fact this is the minimum stated in the Steam site. It appears GoG has artificially inflated the minimum requirements of the game for their installer unnecessarily.
All that said, the trick is to "unpack" the package and extract the game bundle from it manually. I followed instructions posted somewhere else, but here they are for your convenience:
1. Download the installation package from your library.
2. Figure out how to unpack the package. There is a command-line option to do so, which I don't know, or you could download
Pacifist like I did, which does it for you.
3. Once unpacked, browse the contents of the Resources looking for a folder called simply "payload." If you are using Pacifist, this will be in the "Resources" tab:
- thimbleweed_park_en_1_0_866_11225.pkg
|--- Resources
|------- package.pkg
|----------- Scripts
+--------------- payload
4. Extract the "payload" folder. If you are using Pacifist, just select it and click "Extract To..." from the tool bar.
5. Once extracted, find the "payload" folder in your computer and rename it to "Thimbleweed Park.app" -- notice that the extension of ".app" is important. This is the "magic" that causes your computer to treat the folder as an application.
6. Copy the renamed bundle to your Applications folder or wherever you want it.
Ta-daa! You're done! The game can now run as normal.
The fact that the game runs perfectly fine by extracting it manually from the installation package, but the installation package refuses to install it, suggests that there is a problem in the installation package itself.
Good luck and godspeed. :)
-dZ.