Thanks for reporting this issue. I am able to install Copilot4Eclipse from the Eclipse Markteplace using the Eclipse Marketplace Client in Eclipse. I also tested installing directly from the https://www.genuitec.com/updates/copilot4eclipse update site with no issue. The http 504 is common for directly accessing the updatesite from a browser.
Can you share how you are installing Copilot4Eclipse and receiving an error?