Hi Marcos,
If you want full operational seamless integration for A2A and B2B communications, go for SAP PI. SAP NetWeaver Gateway could be used for creating REST-based services with simple configurations, but from Middleware perspective SAP PI is far more better solution.
Check this document for more clear idea:
A simple overview on SAP Netweaver Gateway
REST services with Gateway and PI, what do we need to know?
Regards,
Nabendu.