In the example vtgate-up.sh script you'll see the following lines:
-mysql_server_port $mysql_server_port \ -mysql_server_socket_path $mysql_server_socket_path \ -mysql_auth_server_static_file "./mysql_auth_server_static_creds.json" \
In this example, vtgate accepts MySQL connections on port 15306 and the authentication information is stored in the json file. You can then connect to it using the following command:
mysql -h 127.0.0.1 -P 15306 -u mysql_user --password=mysql_password