First, many people respond differently to a task depending upon whether it is assigned by others or self-initiated. Think of the difference in the kind of reaction you might provoke when forcing someone to eat as compared to inviting the same person to have a taste of a food he might like.
Second, if video games "provide" immediate gratification, why do so many people avoid playing, stop playing quickly, or report very little pleasure when they play? Depending upon many different factors, people may enjoy a particular activity or dislike it (even if the people designing the activity put in a great deal of effort to make it pleasurable).
Third, activities such as schoolwork can "provide" instant feedback, but people might still avoid, give up quickly, or rush to finish. And many people can be fully aware of the negative longer-term consequences of their actions, yet still continue to behave in those ways (e.g., smoking, over eating).
An alternative explanation: Depending upon a child's history of reinforcement, some tasks will trigger ADHD responding. But it has nothing to do with a task "providing" instant feedback. Tasks associated with success and personal control are unlikely to trigger ADHD, while tasks associated with adversity, failure, negative evaluation, and loss of authority are more likely to evoke ADHD reactions.
Yes, you can give a child extra rewards and threaten punishment, and those changes can get ADHD behavior to stop. But that does not mean that the absence of those "extra consequences" causes ADHD responding to occur.
Helpful tip:You can also eliminate ADHD by increasing success with a task so that it triggers pleasurable associations. Threatening or offering a gift reward is not your only option.