You need to be logged in into Xero organization to be able to get project tasks.
Select from the menu Add-ons -> G-Accon for XERO -> Reports Designer -> Design Projects Reports.
Select Tasks and the fields you want to be pulled into Google Spreadsheet. The list of all task fields and descriptions can be found at Xero API documentation page.
The date range filter will be applied against the project deadline date. Addition filtering can be done by project name, contact name, project status, and task name.
By default, the task list is sorted by project deadline date. The task list can be sorted by contact name, project name, project status, and task name.
Xero tasks list example: