Async and GUI threads are different concepts. Of course most GUIs have an event loop which can be used as a form of async, but with async you do your calculations in the main thread, while with GUIs you typically spin your calculations off to a different thread.

Most often when doing async you have a small number of tasks repeated many times, then you spin up one thread per CPU, and "randomly" assign each task as it comes in to a thread.

When doing GUI style programming you have a lot of different tasks and each task is done in exactly one thread.