Britain's most expensive goalkeeper, signed from Hearts by former Black Cats boss Roy Keane for £9million in 2007, has fallen behind Marton Fulop in current manager Ricky Sbragia's pland.
Gordon has also lost his place in Scotland's starting XI due to his lack of domestic action, and the shot-stopper is considering his options.
He told the News of the World: "It is the first time this has happened in my career. It is very difficult to be patient. There is frustration and anger. It is horrible just sitting there.
"It's not about ego. If your ability makes you the best, if your technique is the best, then you should be playing. I am still very confident that I can be the No.1 and I will give a good account of myself the next time I get a chance.
"But it has been a very difficult thing to face up to. I have sat in bed some nights until two or three in the morning not being able to sleep.
"I've had nights where I've not slept at all, thinking, 'what am I going to do? Who can I speak to?' There's no off switch. You just have to find a way to deal with it.
"In the dressing-room before a game is probably the hardest point. The guys are getting ready to do battle but you're not involved.
"There are definitely going to be problems at some point. I want to get back in the first team but I would eventually consider my future if it came to that. There is not a lot I can do about it right now."