How To Set Order State Completed after Notification?
Unfortunately, the &quot;Default&quot; order workflow for Commerce 2.x is fairly naive. It only has states for Draft (i.e. shopping carts), Completed (i.e. any placed order), and Canceled. If you need more states, you'll need to supply an alternate workflow. Core ships with a &quot;Default, with validation&quot; workflow you can set for your order type that includes a &quot;Validation&quot; step you could make use of instead.
Generally speaking, it's likely your payment gateway module shouldn't be controlling the actual order state transition. It can and should update the payment transition, and then a separate module specific to your site can be responsible for defining exactly what you want your workflow to be and automating the transition based on the order paid event (cf. OrderEvents::ORDER_PAID).