There is a Mac version available elsewhere, but not on GOG. Is the Mac version coming to GOG?

Even though the store page says it's Windows-only, I can install it in GOG Galaxy on a mac. However, it doesn't start, saying: "The Case of The Golden Idol.app" is damaged and can't be opened. You should move it to the Trash.

I can also download the offline installer, but it can't be used at least in macOS Sonoma (14.2) as the installation package isn't signed properly: “the_case_of_the_golden_idol_enUS_2023_08_30_dlc_2_0_1_.pkg” can’t be opened because Apple cannot check it for malicious software. This software needs to be updated. Contact the developer for more information.

As you pointed out, the mac version is indeed available elsewhere. This appears to be yet another game whose GOG version is inferior comparing to that on other platforms.
It's quite discouraging, really. They keep talking about creating a community around games, but it seems to be yet another instance of empty words to attract sales, nothing more.

I often install apps from other sources on my Mac, so I think I had some of those precautions disabled beforehand.
I just Installed the game and both dlcs, just had to do a force start of the app the first time I launched it (right click and holding command key while clicking "open"). MacOS 14.2.1 here.
Game launches into the menu without issues.

And yes, of course I would prefer a more seamless approach.

You can use the offline installer (.pkg) by right-clicking on it and selecting "Open". After installation, you can start the app by right-clicking on it and selecting "Open". You only have to do this once, after that it will run. On macOS 14.3 it runs without problems. It is a universal app too; it runs natively on M-series chips.

The installer package is code-signed by GOG, but it is not notarised by Apple (which is a requirement on current macOS versions). The app is code-signed as well, but the seal is broken, because two files were added to the bundle after it was signed by GOG:
codesign -vv The\ Case\ of\ the\ Golden\ Idol.app
The Case of the Golden Idol.app: a sealed resource is missing or invalid
file added: The Case of the Golden Idol.app/Contents/Resources/goggame-2094481873.info
file added: The Case of the Golden Idol.app/Contents/Resources/goggame-2094481873.hashdb

I believe it is alright to override those warnings to install the app. That said, GOG should really learn how to properly construct installer packages and use the proper code-signing procedure. It is not rocket science.