Checking to see if the field contained "Vancouver" and failing validation if it does.

Gravity Forms Custom Validation Example

function my_custom_validation( $validation_result ) {
    $form = $validation_result["form"];

    // Sorry, you don't get to live in Vancouver...
    
    if ( $_POST['input_1'] == 'Vancouver' ) {

        $validation_result["is_valid"] = false;

        foreach ( $form["fields"] as &$field ) {

            // NOTE: Replace 1 with the field you would like to validate
            
            if ( $field["id"] == "1" ) {
                $field["failed_validation"] = true;
                $field["validation_message"] = "Sorry, you can't enter Vancouver here!";
                break;
            }
        }
    }

    // Assign modified $form object back to the validation result
    $validation_result["form"] = $form;
    return $validation_result;
}
add_filter( 'gform_validation_[your form ID]', 'my_custom_validation' );