Sortd Lists are not directly associated with Gmail labels, therefore unfortunately you cannot filter email into Sortd lists in the same way that you might filter emails into a Gmail label.
Sortd lists are not the same as Gmail labels although Sortd will create a sub-label beneath the Sortd main label for each list created. You can then create a Gmail filter to place emails into these Sortd list sub-labels and make use of the Sortd import facility. If you add an email to a Sortd list sub-label manually or by filter Sortd will import it into your list.
Note: If the Sortd sub-labels do not exist, editing the Title of your list names (Double click the title to Edit it and then press Enter (i.e. No change is required)) will cause Sortd to recreate the sub-labels.
Sortd does this in the background with no additional input from you, but as it is done on a periodic basis (once every 30 minutes if you are online using Sortd). Import only works with Sortd sub-labels and not with Gmail labels in general (even if you create a label and list with the same name). Currently Sortd will only import 20 items into a list in any given import iteration and will only import into a Sortd list if there are less than 100 active tasks in the list (Completed items are considered Active until deleted or archived). This is to prevent the lists from becoming overwhelmingly large (and to prevent filters being defined in Gmail that swamp the lists). Once Sortd has imported the email into a list it will remove the list label from the email, although the main Sortd label will remain.
This is an often requested feature and we may look to enhance it in the future, but there needs to be some balance between ease of adding to lists and actually ensuring that tasks in the list are actually relevant, useful and up to date.
Personal opinion: Lists really should contain tasks that can be considered active work otherwise the List just becomes swamped and then you may as well have left the emails in your inbox and Sortd will lose its benefit to you. Obviously we all have exceptions to this rule ;-)