How to get rid of annoying Non-secure Jabber Login popup?
Normally you've to just turn on the 'Use SSL' option (Preferences->Accounts->Google->Server Settings).

It's greyed out, because the option 'Automatically find server and port' is enabled. If the option is greyed as well, disable temporary the account then disable the option, then set the manual settings or try to click on 'Use Default Settings' option. You can use either 5222, 5223 or 443.