Gearbox had the easiest job in the world.  
 
When they bought the rights to DNF from 3DRealms and said they'd put it out within a year, everyone knew what they were doing: bug-test what 3DRealms had put together, and release it.  No new content unless it was absolutely necessary.   Just get what was done out the door to close the book and the longest-running joke in video game history. 
 
If the game ended up being good, Gearbox can take all the credit.  If it's bad, blame it on 3DR.  Gearbox WILL come out with a new Duke game in a few years, and they will be judged on that game's quality.  But Duke Nukem Forever will not be seen as Gearbox's game, and it's not their fault that it sucks.