How do I use shortcodes outside the post editor, such as in theme template files or widgets?
For theme or template files, use the function
<?php echo do_shortcode('[[gcal]]'); ?> with the same attributes you’d use in the post editor.
Read this post for a simple way to use shortcodes in widgets.
If you use do_shortcode() or add shortcodes to widgets, you’ll need to explicitly enqueue the plugin’s styles and scripts as well.