Currently, the only way to get information from an API developed with the Gateway Builder is using the OData $metadata parameter. Another popular option for describing and discovering a RESTful service is Swagger, now part of the Open API initiative (of which Microsoft is a member, but looks like SAP is not (yet)).
I saw on the OASIS standards committee web page that Swagger will be an alternative to describe OData metadata in JSON format: https://issues.oasis-open.org/browse/ODATA-907
When will SAP support this?
BTW, I found a node module that can be used to convert an existing OData metadata to the Open API format: https://www.npmjs.com/package/odata2openapi