you may use mark packet (mark udp and tcp 53 port)and then create simple queue with this mark an unlimited bandwidth at top of your users queue
I do not think it will limit www.gmail.com
traffic to the specific amount of bandwidth, neither port 80/443 will give a result.
Firewall/Queue does not offer option to limit by exact domain name, the most recommended solution to do the bandwidth limitation for www.gmail.com
- find all IP addresses used by www.gmail.com
- add them to address-list;
- setup mangle to mark by address-list;
- setup queue for mangle mark.