The game saves are located in ~/Library/Application Support/THQNordic/Desperados
In most cases – if the save data has the same format between platforms – you can simply copy these files to the corresponding location on Windows, even if Galaxy doesn’t support syncing the save data.
It seems that the developer won’t support Catalina. Mac support is no longer mentioned on the GOG store page and the corresponding Steam page shows an incompatibility warning for Catalina. Pretty sad that the “modernisation” in 2018 was so short-lived.