ONDC stands for open network for digital commerce. A GoI initiative to unbundle e-commerce from a platform into a protocol for buyers, sellers and logistics and technology service providers.
A gateway is a node that multicasts search queries to Seller Apps on ONDC, collects results and returns them unfiltered to buyer apps.
There are two critical considerations:
A gateway built on a public blockchain should do this such that anyone can programatically verify it.