You do not need to show the lookup column values within the task list.
The details are available already within the customer lookup value for the created task.
This value includes an ID and a title, whereby the ID is the list item ID of the selected customer record.
Within your customer gallery you could create an icon, which navigates to a different screen with a gallery connected to all tasks.
In order to know which records should be shown there you can 'carry' this information with you.
Navigate(ScreenwithTaskGallery,ScreenTransition.None,{CustomerSelectionID : ThisItem.ID , CustomerSelectionTitle: ThisItem.Title})
For the screen with the gallery showing the tasks you can now make use of a filter (lookup would return one record, not a table of records).
Filter(ListofTasks, CustomerField.Id = CustomerSelectionID )
Please note, that this is just meant to point to a potential, general approach.
You would run in delegation issues with the above setup, risking to not have all tasks been shown.