Head gasket

Overheated, maybe (heat gauge on dash broken) towed home. No water seen in radiator, reservoir empty. But only took a quart of water. Started up, water leak front center. Added stop leak to radiator, started it up. Ugly gooey stop leak leaking under reservoir. Added antifreeze to radiator and reservoir. Let it idle five minutes. No leak but idling kept making noise and reservoir bubbling. Googled someone who said it could be any of four problems, radiator leak being the less costly. Now I am told it may be the head gasket. That sounds bad. If it is the head gasket, can you tell me what to look for or what the symptoms are? Thank you