When you launch a Notebook, it runs inside a container preloaded with the notebook files and dependencies. See the list of containers that Paperspace maintains here.
Custom containers feature lets you pull your own image from Docker Hub. This article will help you prepare a custom Docker container to use with Gradient, show you how to bring that Container into Gradient, and create a notebook with your custom container. We recommend using Docker to get the container image from your system to Gradient.
See a tutorial on using custom containers here.