Same problem. Issue solved.
follow the instruction in following link:
http://wpdocs.philderksen.com/google-calendar-events/getting-started/api-key-settings/?utm_source=gce_lite&utm_medium=settings_link&utm_campaign=docs#content
add your new google API key ‘General settings” of Gcal events.
Refresh your WP cache.
and it will work fine…
