Broken Sword 5 says "not yet available" for install on Mac - anyone can help?

I'm sure that there is a simple solution to this but I can't see anything about this anywhere.

The only place that I have seen the message "Not yet available" is when using GOG Galaxy. If you are using GOG Galaxy, there are select titles that have not yet been optimized for use on the platform. As an alternative, you can simply download the game directly from your account page on the website. Once it's been installed, you can more than likely add it to your Galaxy game list manually.

If you have any other issues, or if this isn't the problem you are experiencing, you might want to contact GOG support.

Good luck! Be sure to share the resolution to your problem here. It may help someone else in the future.

You are correct - I did check with support and it's a problem with Galaxy:
"If a game is listed as "not yet available" in Galaxy, this means that it hasn't been configured to work with Galaxy yet. I apologize for any inconvenience caused.

Until we can make this game compatible with Galaxy, please download and install it through other methods. These are:

1) Downloading the installation files with Galaxy. Installation files use a different delivery method, and are usually smaller than the files downloaded during "installation". To do this, find the game in your Library in GOG Galaxy, and click MORE -> DOWNLOAD EXTRAS, then click the appropriate Game Installer file,

2) Using the GOG Downloader. You can find it here: https://www.gog.com/downloader . Once downloaded and installed, please find your game on gog.com in your library, and click MORE -> GOG DOWNLOADER LINKS,

3) Downloading with your browser - this isn't very recommended, as it offers no pausing/resuming, and no error protection."