First of all, keep in mind that you do not actually have to install games through GOG Galaxy - you can just use it as a download manager (under MORE -> Extra downloads), to get the standalone installer files.
Alternatively, you could try download your games using the old GOG Downloader. You can download it here.
Install it, open GOG Downloader and log in to your account.
Now go to your Account, select the game, click "MORE" and "Get GOG Downloader Links". You will be moved to the GOG Downloader links section of this game, where you should select the files of your choice (some games may have various game-specific setup files). The GOG Downloader will launch, and start downloading. After the download is completed, please double-click on the game in the application's window in order to install it.