[zedlopez]

As another ADHD-inattentive-type person, I think what I'd want to model if I were trying to model it would be that it's unpredictable which of wandering attention or hyperfocus would apply. My noodling here is more about what (at least one person's) ADHD is like and less about fitting it into existing GURPS infrastructure.

So for IQ/Per tasks involving prolonged attention, like Research or doing stake-outs or sentry duty with Observation, or sense rolls for passively noticing something, you make a roll (against Will with a task difficulty modifier for how engaging the character would find the task, maybe) where failure means a penalty to the subsequent roll and success means a bonus.

With maybe different costs for different likelihoods of one vs. the other (or the size of the bonus/penalty). With maybe a possibility of modifying quirks or perks that you're always single-minded or absent-minded in some narrow realm.