Poco

This plugin installs a widget for showing a Google Calendar agenda on the sidebar. 
Once installed it adds a sidebar widget called ‘Google Calendar’ that may be dragged into your sidebar. 
Each widget can be configured with a ID of the calendar feed, a title, and the number of agenda items to show.
The calendar feed ID is the ID next to ‘Calendar ID:’ in the Google Calendar settings.
Multiple widgets can be used on the same page and each one can reference a different feed.
See also <a href="http://notions.okuda.ca/wordpress-plugins/google-calendar-widget/" rel="nofollow ugc">http://notions.okuda.ca/wordpress-plugins/google-calendar-widget/</a>

Google Calendar Widget

This plugin installs a sidebar widget that can show the upcoming events from a Google Calendar feed.

veleno

GCal Events List retrieves events from Google Calendar and shows data in a widget.
To make GCal Events List work properly, you have to activate the plugin and paste your Google Calendar API key and a public calendar ID in the widget admin panel.
It requires some other options. Here is the full list of params:
<ol>
<li>Title (widget title)</li>
<li>Calendar ID (you can find it in the option panel of your Google Calendar)</li>
<li>Google API key</li>
<li>Start from (a valid date in YYYY-MM-DD format)</li>
<li>Max results</li>
</ol>
For a detailed description of calendar settings, read the <a href="https://developers.google.com/google-apps/calendar/" rel="nofollow ugc">Google Calendar Data API documentation</a>
REMEMBER: your Google Calendar must be public, otherwise the widget won’t show any event!

GCal Events List

GCal Events List retrieves future events from a public Google Calendar and shows data in a widget.

Google Calendar Widget vs GCal Events List