order_fulfillment:
 id: order_fulfillment
 group: commerce_order
 label: 'Fulfillment'
 states:
   draft:
     label: Draft
   awaitingpayment:
     label: Pay Later (Furikomi)
   fulfillment:
     label: Fulfillment
   completed:
     label: Completed
   canceled:
     label: Canceled
 transitions:
   place:
     label: 'Place order'
     from: [draft, awaitingpayment]
     to:   fulfillment
   paylater:
     label: 'Pay later'
     from: [draft]
     to:   awaitingpayment
   fulfill:
     label: 'Fulfill order'
     from: [fulfillment]
     to: completed
   cancel:
     label: 'Cancel order'
     from: [draft, fulfillment]
     to:   canceled

public function onPlaceTransition(WorkflowTransitionEvent $event) {
    $order = $event->getEntity();

    $order_type = $order->bundle();

    $payment_method = NULL;
    $payment_gateway = NULL;
    if (!$order->get('payment_method')->isEmpty()) {
      $payment_method = $order->get('payment_method')->first()->entity->id();
    }
    if (!$order->get('payment_gateway')->isEmpty()) {
      $payment_gateway = $order->get('payment_gateway')->first()->entity->label();
    }

    if ($order_type === 'ABC') {
      // https://drupal.stackexchange.com/a/186333/5147
      if ($payment_gateway === "Pay Later") {
        // https://drupal.stackexchange.com/a/252624/5147
        // Set order status to "payment pending"
        $order_state = $order->getState();
        // Get the allowed transitions for the current state.
        $transitions = $order_state->getTransitions();
        $order_state->applyTransition($transitions['paylater']);
        $order->save();