This sort of catch-all-phrase refers to a general, overall state of anxiety that is not necessarily related to any specific event or circumstance such as due to a medical condition. Listed under 300.02 in the DSM-IV Manual, GAD, in my observation, should be an almost automatically included component of any neuro-biological disorder.   
I'm speaking here of a general anxiety component to neuro-biological disorders and not a specific condition such as Panic Attacks. In my view anxiety is an "anxious concern" that the "anxiety" (which is not related to anything specific) is not going to go away, that it will remain around to haunt the person for no good reason. Try as they may, they can't get rid of it. It seems to be a basic component of impulse disorders. Seen in that light, it's easy to characterize a person as anxious due to their often present demeanor of being "uptight," edgy," "worried" and "defensive."   
A solid overview of anxiety and specific conditions can be seen on the Anxiety Disorders Association of America site at:

Tourette Syndrome
Attention Deficit Hyperactivity Disorder
Obsessive Compulsive Disorder
Oppositional Defiant Disorder
Intermittent Explosive Disorder
Bi-Polar Disorder

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