Payment Gateway Integration

Are you perhaps referring to:

http://paymentsgateway.com/

or recently becoming

https://www.forte.net/

If so K2 does have some ootb broker to interact with any endpoints that paymentsgateway may expose:

http://help.k2.com/onlinehelp/k2blackpearl/UserGuide/4.7/default.htm#ServiceBrokers/EndPoints/EndPoints_WEB.htm

http://help.k2.com/onlinehelp/k2blackpearl/UserGuide/4.7/default.htm#ServiceBrokers/K2ODataServiceBroker/K2ODataServiceBroker.htm

http://help.k2.com/onlinehelp/k2blackpearl/UserGuide/4.7/default.htm#ServiceBrokers/EndPoints/REST_Endpoint.htm

If paymentsgateway store back-end data in SQL, perhaps the SQL broker to interact with the tables:

http://help.k2.com/onlinehelp/k2blackpearl/UserGuide/4.7/default.htm#ServiceBrokers/SQLServer/SQL_Server_Service.htm

Looks like they have REST web services:

https://www.forte.net/devdocs/

So perhaps the K2 Rest broker:

http://help.k2.com/onlinehelp/k2blackpearl/UserGuide/4.7/default.htm#ServiceBrokers/EndPoints/REST_Endpoint.htm

Perhaps a custom paymentsgateway broker can also be created to use their APIs:

http://help.k2.com/onlinehelp/k2blackpearl/devref/4.7/default.htm#Custom_Service_Brokers.htm