Gunicorn is one of my favorite app servers. I've been using Gunicorn in production on a bunch of projects (any Flask / Django project I happen to work with) for around 8 years now. It's rock solid and extremely predictable.

I also use Gunicorn in development too, it supports code reloading and also lets you enable Flask's debug middleware so you can use the interactive debugger. This is all configured in my Build a SAAS App with Flask course, a code example is here: https://github.com/nickjj/build-a-saas-app-with-flask.

A Django example is here: https://github.com/nickjj/docker-django-example