Hotspot Walled Garden IP changes not "sticking"

I have a small set up with about 20 users, Hotspot enabled. Most of the users don’t log in to use the internet, but just send/receive email (Google Apps for Biz). To save on their limited internet hours, I added the Gmail incoming/outgoing servers to the Walled Garden IP List. However, when I make changes to the IP address in the IP List, the change is not reflected in the Walled Garden. I’ve tried adding new rules, disabling/enabling, rebooting, and it’s still not working. The router is an RB5xx, and it’s running 2.9.50 (yes, I know I need to upgrade, will do soon).

Any ideas what is going wrong? I’ve been making changes so far via Winbox, but maybe via terminal would be better?

Please help!