The main difference between an PaaS like Heroku and a general purpose cluster manager service like GKE is that the former can simplify my making certain assumptions about your workload.

For example if a service knows your container is serving a web application then it can sensibly provision load balancers, DNS, HTTPS, auto-scaling, static content caching, automatic QPS monitoring etc. to support your app with little explicit configuration. And with a commercial service you get an SLA for those things.

You can get much of this with a general purpose cluster as well, but of course you need to configure it yourself, and more importantly - debug it when it goes awry.

Disclaimer: I work for Google