In order to make sure that your website is delivered without showing a security warning please make sure to add your domain to the GWDG SSL certificate.
The redirection can be configured by adding additional rules to the .htaccess-file, which is processed by the webserver on every request.
A working configuration is shown in this example:
# GWDG SSL REDIRECTION
RewriteCond %{HTTP:X-Forwarded-Proto} !https
RewriteRule ^.*$ https://%{SERVER_NAME}%{REQUEST_URI}
# END GWDG SSL REDIRECTION
Information on activating SSL with Wordpress can be found under Wordpress with SSL