Gary spends much of his day wondering "what if" about catastrophic scenarios. This thought process makes him feel he has control over the situation. However, the lack of any catastrophes only reinforces the "what if?" behavior. Gary would most likely be diagnosed with

Generalized anxiety disorder.

Explanation:

This is practically seen or can be said to be a disorder that makes someone think of so many things simultaneously. It said to be a common anxiety disorder that involves constant and chronic worrying, nervousness, and tension. This is different from a phobia in as much as it seems like it, but in a phobia situation, a phobia gives you fear that is connected to a specific thing or situation, but this disorder general makes you feel of dread or unease that colors your whole life. This is exactly what Gary is feeling in the scenario above. Even though a lot of people tend to gets anxious sometimes, but if your worries and fears are so constant that they affect your daily living, then it is obviously a disorder.

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