I want to ask if I can add a checkout button in shopify plus checkout page that redirect me to payment gateway checkout page (this payment provider does not have HSDK with shopify) and complete the payment in the payment provider then redirect the customer to the confirmation page. is this flow is applicable? Also, what are APIs that should be used on this case to complete the order.
The gateway would need to be installed in your shop for this flow to work.
Since you said they gateway isn't integrated the flow you're talking about would not be possible.