I experienced the same strange loss of sync. Items I entered on my phone were showing up on my desktop calendar, but items I entered on my desktop weren't syncing with my phone. I don't know if it had something to do with the 2-step security feature I recently enabled with Gmail?
In any case, after trying a number of things I figured out how to get it working again, and it's much easier than the old Exchange setup: just turn the calendar off and on again!
Settings>
Mail, Contacts, Calendars>
Gmail (under Accounts)>
Toggle the Calendars On switch to Off>
Wait a moment, then toggle it back On.
After about one minute the Calendar had updated with all missing items!