Gearbox are the ones who finished and released the game. Thus, everything about the game is their responsibility. They may not have build the game from the ground up but if certain unpopular design choices were actually 3DRealms' doing, Gearbox could've easily spotted and changed those. Apparently they chose not to, so yes, all those unpopular aspects of the game design are their fault.
To the people who keep repeating "Oh what did you expect after so much time of course it was gonna suck lol!" like it's some holy mantra: this game would've been received a LOT better if it had simply used healthpacks rather than regenerating health, had no maximum weapons limit, had higher movement speed, and had encounters with more than 4 enemies at once. These things are immediately noticeable to the player, and they're exactly what fans of oldschool FPS hate.
To the people who keep repeating "Oh what did you expect after so much time of course it was gonna suck lol!" like it's some holy mantra: this game would've been received a LOT better if it had simply used healthpacks rather than regenerating health, had no maximum weapons limit, had higher movement speed, and had encounters with more than 4 enemies at once. These things are immediately noticeable to the player, and they're exactly what fans of oldschool FPS hate.