Widget Styling

The widget stylesheet is here:
/wp-content/plugins/gc-testimonials/assets/css/gctwidget.css

To skip loading the stylesheet altogether so you can start from scratch, try this in your theme’s functions.php:

function my_dequeue_styles()
{
  // wp_dequeue_style( 'gctstyles' );  // for shortcodes
  wp_dequeue_style( 'gctwidgetstyles' );  // for widget
}
add_action( 'wp_enqueue_scripts', 'my_dequeue_styles', 99 );