You are assigned. In the future, don't sell a call on option expiration day at the money. Ever heard of "max pain"? The market maker loves collecting off of novice traders. You are now short $17,603 in gld plus the cost of the assignment and any rise after hours. If there is a gap up in price Monday, you may be in even deeper debt. You would have ben better off just shorting the shares and then you could have gotten out of the trade(or stayed in it) on your own terms.