GTasks is a free Android task list app for managing your tasks, to-do lists, check lists, or notes on your Android phone. Sync online to your Google task list for viewing in Google Calendar or Gmail. GTasks offers added functionality over only using Google task list.
Google Calendar
Android Phone Calendar Not Showing All Calendars & Events
Your Android phone can show calendars from your Google account’s Google Calendar. If you have multiple individual calendars setup in your Google Calendar account, you may suddenly find that some calendars are no longer showing events on your Android phone.