I figured it out with additional code. Works like a charm.
add_filter( 'gform_get_form_filter_4', 'custom_schedule', 10, 2 );
function custom_schedule( $form_string, $form ) {
$start = '07:00:00';
$end = '19:00:00';
$now = date("H:i:s");
if($start < $end) {
// if current time is past start time or before end time
if($now <= $start || $now > $end){
$form_string = '<p>We are closed today, please return tomorrow to make your
booking.</p>';
}
}
// else time frame is within same day check if we are between start and end
else if ($now <= $start && $now >= $end) {
$form_string = '<p>We are closed today, please return tomorrow to make your
booking.</p>';
}
return $form_string;
}