With a GTC (Good Til Cancelled) listing, ebay relists your item automatically every 30-day period from the time you create it until all the items in the listing sell, you end the listing, or ebay ends the listing. Each time it relists it will be counted against your monthly free listing limit and if you passed that, it will charge you an insertion fee As of mid-March 2019, all fixed price listings are automatically GTC.
If your listing has a quantity of 0 for the entire 30-day billing period for 3 consecutive 30-day billing periods, it will be ended permanently.