The line above is from WSA GUI -> Security Services -> End-User Notification -> HTTP General Settings under the "Logo Image" section. And looks like your configuration is using "Use Cisco Logo" and the ip address before :8443 should be your M1 interface IP address.
This logo will only be used when the WSA need to display notifications such as block notification, warning notification, etc. If the traffic to gmail is allow then WSA will not need to display any of its own notification.
I am suspecting that the traffics to gmail are not fully allow and some domains that gmail is using might be set to block/warning, etc.
What is the accesslogs from WSA shows you?