At some point, the only way to use Google Calendar with Apple's Calendar.app was to set up two-factor authentication and then create an "app-specific password" to use in the application. On my laptop, running Mavericks, this quit working a couple of days ago. Trying to create and enter a new password didn't work either. Eventually, I used this as an excuse to install El Capitan where the Internet Accounts System Preferences now directs you to authenticate directly via Shibboleth to configure a Google Account. But I suspect this means that Mavericks (and previous) systems will find it hard to work with Google anything going forward and will need to be updated to Yosemite (or El Capitan).
Science educator, biologist, technology guru, and award-winning author of Esperanto-language haiku, haibun, and prose. he/his