Hi @anon50705889, welcome to the Asana Community Forum! As it stands, it’s currently not possible to automatically create subtasks from the Gmail add-on. I recommend you voting for this feature. here: Feature Request: Making Subtasks in GMail Plugin.
As a workaround, you can try to use Zapier to trigger an action when new tasks are added to the projects > a list of subtasks is added:
I hope this helps!