Mule API Gateway vs Mule Runtime
Pretty confusing to me at first, as i would have thought mule Runtime host mule API Gateway. While that's definitely not the case, API Gateway connects with API Manager to enforce and apply policies / settings like throttling, security, CORS to your back end services. These enforcement are based on sa specific apps.
Mule Runtime is where all your application gets hosted and run. It takes incoming request, runs the specific flows and returns results.