Old GP4s (400gr) would definitly be a good rim choice for cyclocross tubulars. I would be hesitant to use the similar but lighter GL330 for cyclocross, probably not durable enough for offroad. Box section tubular rims should be a bit softer riding than aero V profile rims, good for cross. I think that any aerodynamic benefits of V profile rims at cyclocross speeds are really, really minimal and V section rims are generally heavier. The one benefit of V section rims however is that they can shed mud better than box section rims.