seperate Api keys for different backend services on GCP

Hi Guys, 
I am trying to use separate API keys for each backend service authentication on GCP using swagger2.0 (GCP supports only OpenAPI 2.0 as of now) however I am not able to put different API Keys in a config YAML of API gateway, can someone help me to share how can I do it, also I want to use authorization for that. Thank you.