This link here directs you to the eBay Support pages and it covers an explanation of what Good Til Cancelled means.
https://www.ebay.com.au/gds/What-is-a-Good-till-Cancelled-listing-/10000000175021366/g.html
Listings that come from eBay LINK:
When a listing is created on eBay, it is created as a Fixed Price, Buy Now listing that is 'Good til Cancelled'
We do not create listings with the auction format.
This FAQ here covers listing prerequisites and more info about listings on eBay - http://ebaylinksupport.omnivore.com.au/support/solutions/articles/17000061466-how-to-list-a-product-on-ebay