I found this -
So I tried accessing an endpoint via the attached image method in postman and it looks like we are on the right track. Looks like the arguments are as follows -
"contact_info" => $contact_info, "amount" => $amount, "currency" => $currency, "invoice_amounts" => $invoice_amounts, "options" => $options, "gateway_id" => $gateway_id
This method and docblock can be found in \components\gateway_payments\gateway_payments.php
/**
* Returns an array of HTML markup used to render capture and pay requests for non-merchant gateways
*
* @param array $contact_info An array of contact info including:
* - id The contact ID
* - client_id The ID of the client this contact belongs to
* - user_id The user ID this contact belongs to (if any)
* - contact_type The type of contact
* - contact_type_id The ID of the contact type
* - first_name The first name on the contact
* - last_name The last name on the contact
* - title The title of the contact
* - company The company name of the contact
* - address1 The address 1 line of the contact
* - address2 The address 2 line of the contact
* - city The city of the contact
* - state An array of state info including:
* - code The 2 or 3-character state code
* - name The local name of the country
* - country An array of country info including:
* - alpha2 The 2-character country code
* - alpha3 The 3-cahracter country code
* - name The english name of the country
* - alt_name The local name of the country
* - zip The zip/postal code of the contact
* @param float $amount The amount to charge this contact
* @param string $currency The current to charge
* @param array $invoice_amounts An array of invoices, each containing:
* - id The ID of the invoice being processed
* - amount The amount being processed for this invoice (which is included in $amount)
* @param array $options An array of options including:
* - description The Description of the charge
* - return_url The URL to redirect users to after a successful payment
* - recur An array of recurring info including:
* - start_date The date/time in UTC that the recurring payment begins
* - amount The amount to recur
* - term The term to recur
* - period The recurring period (day, week, month, year, onetime) used
* in conjunction with term in order to determine the next recurring payment
* @param int $gateway_id The ID of the nonmerchant gateway to fetch,
* if null will return all nonmerchant gateways for the currency
* @return array An array of HTML, indexed by the gateway name
*/