Hmm, that's really puzzling for sure. Let's try with a fresh no-steam version. I will PM you a link to a DRM-free copy of both of them. Only difference between them and the Steam versions is that they don't log achievements to steam.
Regarding gatekeeper, that's going to come up with the drm-free version downloaded through a browser, so here are some notes on that:
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If you look inside the Player.log and find that it's filled with errors about "app translocation," and unauthorized access exceptions and missing files, then that's gatekeeper coming in.
Basically, Gatekeeper on OSX has been told by Apple to move the Whatever.app executable folder into another location, which is randomized and read-only. This is the "GateKeeper Path Randomization" feature.
In the case of all our applications, we have more than just the one folder. Many osx apps run with all of their files in just one .app structure, so this sort of solution would work -- if you also are writing to the designated user data folder, and not the local randomized read-only path. So that's two "ifs."
In our case, as with many games, we have other files and folders next to our games that we read from to get images, configuration, music, and so on from. We also write back out configuration into the same location, rather than to the user data folder. We find that this helps with portability -- players can throw the game on a thumb drive and not be missing anything.
There is a super simple fix to this, though! If, through Finder, you copy the downloaded zip file to any other location other than Downloads or Applications prior to unzipping it, then it should start working. This particular feature of GateKeeper is apparently triggered in the new versions of OSX specifically when an application is downloaded from a web browser and then uncompressed, and then dragged to the Applications folder or run from its current location. They have a hidden flag set on them. Applications being downloaded through Steam or the Mac App Store don't have this issue.
For some more info relating to this:
https://github.com/potionfactory/LetsMove/issues/56Here's perhaps an even better source, although it's much older and interestingly talking about Sierra and not High Sierra:
https://eclecticlight.co/2016/06/16/macos-sierra-will-break-many-installers-and-updaters/TLDR:
1. Download the .zip file to the downloads folder.
2. Move the .zip file to some other location in the main disk drive.
3. Uncompress the zip file there, and it should "just work."