Unable to configure GCP API Gateway where endpoint is outside of GCP - api

Question

I am trying to configure API Gateway on GCP console where the APIs are exposed at external web URL endpoint. I wish to fetch them and store the results in json in GCP cloud storage bucket. So the flow to retrive info is - External URI endpoint -> API Gateway (gcp) -> gcp storage bucket
Below is the URL that has exposed the APIs. I have the key and the element information. I am able to retrive the values via Power BI as well as curl command but I am unable to configure the API successfully on the GCP API Gateway service to fetch the same.
URL - https://xxxx.xxxxxxxx.com/cost/api/organizations/control-elements
Below curl command works -
curl -H X-User-Access-Token:xxxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx https://xxxx.xxxxxxxx.com/cost/api/organizations/control-elements
Below is the openAPI specs file that is being provided on the console
# openapi2-functions.yaml
swagger: '2.0'
info:
title: API_ID description
description: Sample API on API Gateway backend
version: 1.0.0
schemes:
- https
produces:
- application/json
host: 'https://xxxx.xxxxxxxx.com/cost/api/organizations/control-elements'
paths:
/control-elements:
get:
summary: Greet a user
operationId: control-elements
security:
- api_key: []
responses:
'200':
description: A successful response
schema:
type: string
securityDefinitions:
api_key:
type: "apiKey"
name: "xxxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx"
in: "query"
Error message -
Cannot convert to service config. 'location: "unknown location" kind: ERROR message: "Unable to parse the content. while parsing a block mapping\n in 'reader', line 2, column 1:\n swagger: '2.0'\n ^\nexpected , but found ''\n in 'reader', line 11, column 98:\n ... xxxxx/xxxxx/control-element's\n ^\n\n at [Source: (StringReader); line: 11, column: 98]" ' com.google.apps.framework.request.BadRequestException: Cannot convert to service config. 'location: "unknown location" kind: ERROR message: "Unable to parse the content. while parsing a block mapping\n in 'reader', line 2, column 1:\n swagger: '2.0'\n ^\nexpected , but found ''\n in 'reader', line 11, column 98:\n ... xxxxx/xxxxx/control-element's\n ^\n\n at [Source: (StringReader); line: 11, column: 98]" '
Attached is the screenshot of the values entered in the API Gateway service. Please advise on what is being missed over here
Data feed for API gateway service is as shown in screenshot
I tried altering the config in various ways. My service account has APIGateway-Admin access too. Curl as well as through Power BI, I am able to fetch API information from the mentioned URL too