In IC8/CC4, the Smart Gallery is integrated into the Content Manager and called Smart Content Manager. Basically, the user can't install content via the Downloader (installcontent.rlcontent), nor the legacy .exe installer. This can only be done through the Smart Content Manager. See this manual on how to download the content: https://manual.reallusion.com/content-manager/2.0/04-downloading-contents/downloading-contents.htm
You can also review this FAQ to bring over previous version content into IC8/CC4: - https://kb.reallusion.com/Product/53040/Im-an-iClone-7CC3-user-how-do-I-merge-my-old-content-library-with-the-new-iClone-8CC4-library-after-upgrading