If you are going to set up SSL between Access Gateway and the web servers, you need to configure your web servers for SSL. Your web servers must supply a certificate that clients (in this case, Access Gateway) can import. See your web server documentation for information about how to configure the web server for SSL.
For mutual SSL, the proxy service must supply a certificate that the web server can trust. This certificate can be the same one you use for SSL between the browsers and the reverse proxy.