So I'm not an expert, but no one seems to be stepping up to discuss gordos so I thought I'd step up to the plate.
Gordos are potentially B-tier, at least C-tier, but not D-tier. They're too useful for that.
First, the cons of Gordo, which are well documented at this point but for the sake of discussion warrant repeating:
-Easily reflected. This is problematic if Dedede is too close to reflect them back, but reflecting them back is a big part of Dedede's more advanced gameplay, so it's a mixed bag.
-Terribad endlag. Pressing side-b is a 64 frame commitment, and if the opponent hits you out of the animation before you can launch the Gordo (frame 29), then Dedede is unable to interact with it (but his opponents still can because Sakurai hates you). Dedede needs to be on the opposite side of the stage from his opponent to have enough time to reflect gordos back.
-Janky and inconsistent. Sometimes gordos will bounce off you twice for reasons I don't fully grasp. Sticking them to walls is incredibly powerful, but they only do it half of the time because no, really, Sakurai hates you.
With that in mind, though, gordos are still hugely useful.
-Long lasting hitbox with high damage. Let's not forget this part. Gordos are big, hang around forever, and do 14% up close and 11% late, with high knockback.
-Excellent for landing traps. Dedede can use gordos to cover one landing option while he covers another. Putting a Gordo on the Smashville platform shuts down one of the safest ways to land on that stage.
-Combo starter/extender. Opponents hit from behind by Gordo get launched back toward Dedede, who can follow up. Gordo into uair does a dededisgusting amount of damage and can lead to early kills.
-And of course, Gordo is one of the safest and most effective ledge trapping tools in the game. A well placed Gordo can cover almost every ledge option, including doing nothing. With gordos covering most every option and Dedede covering the rest, getting up from the ledge against Dedede can be a pretty daunting task. And if Dedede actually manages to stick a Gordo to the wall while your offstage, God help you.
With all that in mind, I think it's fair to say Gordo gets a bad rap. Yeah getting Gordo reflected back into your face is embarrassing and sucks, but it's way too powerful and versatile a tool to be D-tier. My gut instincts say it should be B-tier, but I'm probably biased because I main the bird, so if others see it fit for C-tier I wouldn't see a problem with that.