You can create HubSpot payment records via API, but I would not recommend using workflows alone for this setup. Since you have 6 external NMI gateways, the best structure is to use a middleware/API layer that receives all gateway webhooks, normalizes the data, prevents duplicates, handles failures/retries, and then creates or updates payment/transaction records in HubSpot via API.
This gives you the closest experience to native HubSpot Payments while keeping NMI as the source of truth.
Then you can use HubSpot workflows based on payment status, gateway name, amount, contact, deal, invoice ID, etc. for downstream automation and reporting.
I’d also strongly recommend implementing proper logging, webhook retry handling, and transaction validation because payment systems at your scale need reliability and traceability.
If this helped, feel free to give an upvote. Also let me know if you need any support with the architecture or implementation.