Gravity Forms - custom field validation function for checkboxes. Ensures all checkboxes have been checked.

gistfile1.php

// Replace 7 with the ID of your form and 13 with the ID of the field you want to force "all required"
// http://www.gravityhelp.com/documentation/page/Gform_field_validation
add_filter("gform_field_validation_7_13", 'validate_tcs', 10, 4);

function validate_tcs($result, $value, $form, $field) {  
  	// Convert the checkbox input name value (returned as part of "field")
  	// into the "underscored" ID version which is found in the $_POST
	foreach ($field['inputs'] as $input) {
		$input_post_value = 'input_' . str_replace('.', '_', $input['id']);
	
		// Validate the value
		if ( !isset( $_POST[$input_post_value] ) ) {
			$result["is_valid"] = false;
			$result["message"] = "You must accept <em>all</em> of the Terms and Conditions";
		}
	}		
	
	return $result;
}