Sku Grid does not End GTC listings, they simply renew every 30 days and if the Out of Stock Option is checked in your ebay account then sku grid sets qty to 0 when out of stock and sets it back to default when back in stock.  GTC items are not allowed to be relisted by the is assumed that if a GTC item is ended that the user wants it ended.  GTC items would only end for one of the following reasons:

1. User ended the item themselves.

2. Out of Stock Option was not checked in the eBay Site Preferences

3. The item was originally listed as a 30 day listing and user attempted to change it to GTC after the fact.  This is technically allowed on the front end of ebay, but ebay only allows the item to be treated how it was originally listed.

So, in order to get these items back up, you must list the items again and make sure that GTC is selected before the items post.  Also make sure that the Use Out of Stock option is checked in your ebay account.  Then, you would need to edit the entries in Sku Grid and replace the old ebay item number with the new one so that it is tracking the correct thing.  They should be okay going forward.