yes you can block gmail and monitor the same but with saome special arrangments(proxies)
plesae refer below links for more details
https://www-secure.symantec.com/connect/forums/symantec-data-loss-prevention-dlp-dlp-monitor-and-https-traffic
https://www-secure.symantec.com/connect/forums/can-we-block-https-traffic-through-symantec-dlp-network-prevent-web
https://www-secure.symantec.com/connect/forums/monitoring-http-get-vontu-dlp-web-prevent