Hello rathnakarreddykasireddy ,
I guess you're trying to publish the document of API on gateway. If yes, you can publish a new service with file name.
for example, you have an API "/test" on gateway, you want to publish its swagger document "/test.json", you may just "Publish Web API" on the gateway with resolution path set as, /test.json
In the policy, you only need to add "Return template response to requestor", the context-type set as "text/json; charset=UTF-8", and in the response body, you input the complete swagger document of the API "test"
Regards,
Mark