Using Safari on iPhone and go to: https://m.google.com/sync/settings/iconfig/ , sign in your google account.
“Manage devices” page will show and devices you’ve synched with google are listed.
Select iPhone in my case, check the calendars to be synced in the following page. Save to confirm changes.
Wait for several minutes for it to sync.
Go back to iOS calendar, click on “Calendars” at the top left, the checked google calendars should show, otherwise, click the “update” button at the bottom left and try later. They will be there.