I'm not interested in the technical solutions to this, they're abhorrent also. With regard to results, timelines and goals all of those are incredibly hard to measure accurately in a product delivery context.
If the persons contract says "will deliver X by Y date" then by all means work the hours required and no more. What this persons contract actually says is "Will work 8hrs a day, 5 days a week, on whatever tasks are assigned. Will be paid $$$". 'Gavin' is not doing this, and he's not even just being a bit flexible. He's actively pretending to work when he's not -- that's fraud / contract violation.