Integration of GK POS & SAP ECC Retail

Hi, I need communicate SAP ECC Retail 1909 with SAP POS GK without CAR neither PO, Ive read that i have to use IDoc/XML for this integration, I have some question is someone can please help me:

- Does the communication with IDoc would be directly without use flat files? or its necessary use flat files for the communication?

- are there an addon that activate some webservice in SAP ECC for this integration?

I would like understand a little more how SAP POS GK works for the integration.

Thanks