Technically not. Can't give you the exact 'book' quote, but it's a case of a new clearance cancels a previous clearance & any restrictions unless otherwise reiterated.
It's only really an early morning thing, we'll always try to keep you as high as possible for as long as we can, and cut as many miles off a route as we can. At that time of day it's not a big deal if you're high, but I'd still always check when you're given the direct, just in case. In a stream if number one complies with GIBSO, and everyone else goes in high, all the lovely speed control fails rather quickly with the different level bands.
I was always lead to believe if you were going direct GWC about FL270 at GIBSO was roughly on the descent profile?