If gearbox release a bad game, they release a bad game. They could have just as easily decided to NOT release it if they thought it was bad. Sure, maybe they didn't make the bulk of the game - but after seeing it, working with it, developing it, they decided to release it, and whether or not the game is good or bad reflects on their ability to judge quality.
I haven't played Duke Nukem Forever, and unless you work at Gearbox or 3D realms, you probably haven't either. I can't say whether or not the game is any good yet. But Gearbox made the ENTIRELY voluntary decision to resurrect the game from death, so they have to live with whatever reception the game gets, positive or negative.
If you take the angle that they bought the game just so that they could get the IP.... that doesn't make sense. They could have just as easily decided to shutter it the day after they bought the rights to the IP. They aren't obliged to release Duke Nukem Forever.
Regardless, even if DNF turns out to be trash, I won't hold Gearbox responsible for making it trash. I'll hold them responsible for deciding to release it as trash, but the fact of the matter is that 3D Realms was responsible for that joke of a development cycle. Hell, that DNF is even being released in a form that DOESN'T resemble "Big Rigs Over the Road Racing" is a monumental triumph!
EDIT: And if you say that they have to release it "as is" because they bought it and it cost them too much for them to NOT release it..... then why did they buy it without checking the quality? To use the chef example: Imagine if a Chef bought some ingredients without even checking the quality of the ingredients to use in a dish, and then finds out that the ingredients are awful. Straight up, he's a bad chef. If Gearbox bought DNF without checking to see what condition it was in and without being willing to do more work on it if necessary, then they're really.... really..... really stupid. Smart people don't buy things without checking the quality of those things first, and if Gearbox bought DNF blind, then that doesn't speak well of their business practices.