I have no problem with ramping up homework load as a child gets older, because a teenager who needs 8 hours of sleep a night obviously has more time to invest than a 7yo who needs 10-11 hours a night.
With that said, a gifted kid (assuming no LDs that impact homework performance time) should be able to find a significant amount of time at school that they can use for the homework. A two-pronged strategy of working with the teacher to reduce homework requirements AND working with the kid to see if he can find more class time to do it would be likely to yield satisfactory results.
As an example, my DD6 brought home a book from her G/T class, and she has to do an assignment on it by the end of the month. Because she procrastinates, I set an aggressive reading schedule for her, which takes up about an hour a day, and replaced our evening bedtime reading (Harry Potter) with the assigned reading. DD has independently responded by reading sufficiently on her own time during school, so she can have her Harry Potter back.
