Flask, Gunicorn and Docker Swarm
Yes, when using Docker Swarm you'll still want to use Gunicorn in your Docker containers.

Gunicorn is necessary to facilitate the communication between the server and your web application. In the case of a Dockerized Flask application this is still the case as Gunicorn handles the communication between the Flask application and the Docker container. Adding Docker Swarm to this stack basically just means that you have more instances of your container, and that Swarm will handle balancing the incoming requests to each instance.