In order to use HTTPS loadbalancer it is required to store SSL certificate to be used in loadbalancers. SSL Certificates can be managed from "Certificates" tab as shown in Img. 1

Img. 1

Click on "Add Certificate" to provide certificate and secret key to be stored in GCloud platform. (Img.2 )

Img. 2

Certificate and private key are mandatory, while intermediate certificate is optional.