I bought Syndicate bus last night. It downloaded and appeared to install okay on my Mac, but I couldn't get it to start. I clicked PLAY and the button turned from green to grey, but nothing happened. After a couple of seconds the button turned green again. No game appeared. I contacted GOG support who told me:

"Since the upcoming macOS 10.15 Catalina update will no longer support 32-bit apps, we expect majority of game developers to update their games available on GOG.COM Store and plan to actively reach out to remind them about it. We will also continue to make sure that system requirements for macOS games on GOG.COM include precise details about the OS version required to run them.

When it comes to DOS games, we are in the process of updating them to use a 64bit DOSBox so they could continue to work."

Since there didn't seem to be anything about this on the forum, I'm posting this. In related news, GOG have offered to give me credit for the game instead of waiting for the update / 64bit DOSBox.

There are a bunch of topics about this on the forums. Also you can just use 64-bit DOSBox yourself; no need to wait for GOG to do it for you.

Thanks for sharing this reply from GOG. I’ve had limited success getting some games to run using a 64-bit version of DOSBox. The problem is that some games use Boxer instead of DOSBox, and though a few 64-bit builds exist out there, implementing them into the executable can prove tricky. Also, I don’t think it’s realistic to expect everyone buying Mac versions to have to fiddle around with this stuff themselves. So, I appreciate hearing about the plan GOG has to update their DOS games. Hopefully it’s sooner rather than later. It would be nice if GOG was also clearer about the games that currently don’t support Catalina. My GOG game library is relatively large and it’s just been a process of elimination to see what works and what doesn’t (in short, very few Mac games actually work on the latest OS).