For the records there are also various existing topics in this portal on how to configure SSL/TLS in gsad:
The relevant command line parameters which needs to be passed to gsad when starting it up (e.g. in the related systemd file) are:
-k, --ssl-private-key=<file> Use <file> as the private key for HTTPS
-c, --ssl-certificate=<file> Use <file> as the certificate for HTTPS