There are many differences between GAD (generalized anxiety disorder) and panic disorder. GAD is characterized by constant worries about everyday events, while panic disorder is characterized by discreet, random, and spontaneous panic attacks. Panic disorder may seem like GAD as when you have panic disorder, you have constant worries about when the next panic attack is going to occur. This is called anticipatory anxiety, as you are anxiously anticipating when the next panic attack is going to occur when you have panic disorder. This is different than the worries in GAD, where your worries are about everyday things like relationships, jobs, and finances, and the worries in GAD are not about anticipating the next panic attack.
Another difference is that in GAD, it is rare to have panic attacks. Also, with panic disorder, you can develop agoraphobia, which is a fear and avoidance of places or situations where you might get a panic attack or where escape may be difficult.