To answer your question, Losant currently does not have a Raspberry PI GPIO Node that you can use in your edge workflow to directly access the GPIO.
I agree that you would be able to use the Losant Edge Agent to control your Raspberry Pi GPIO by having additional code running on the Raspberry Pi (or create another Docker image that has the code in it). If you create another Docker image with your code in it, then when deploying to other Raspberry Pis you would pull down and run this Docker image that you created and the Losant Edge Agent.
If you go the route of another Docker container, you will have to allow this Docker container to have access to the GPIO. Here is an article that shows how this can be done.
Within this Docker container that you create, you can run a webserver that accepts HTTP requests from the Losant Edge Agent which will then read or command the GPIO. It will act as the interface between the GPIO and the Losant Edge Agent.
In addition to this solution, there is a blog post on how to control the Raspberry Pi GPIO with Losant (but it does not use the Losant Edge Agent).
Let us know if you have any other questions and how you end up building this solution to allow the Losant Edge Agent to control your Raspberry Pi GPIO.