Something that I feel is not covered as much as it should be is that when a player is accused of using a "noob gun" they may often actually be using a "noob strat" and then the weapon is unfairly blamed. While much of this discussion can be transferred to "noob strats" as well, there's one specific examples that I want to mention:
I (MG2) once played a team that on CT Mirage bought shotguns every round and hid randomly in one of the two bombsites (the entire team in the same site). Now, at our level their strat was fairly effective when we attacked the stacked bombsite. They had clearly practiced doing this and using shotguns. In addition to this, Mirage was a map that lends itself particularly well to hiding in bombsites. Now, was there a counter to this? Yes, of course. Learning all the hiding spots, learning how to peek them one at a time using the entire team, and learning the relevant smokes and grenades would all have helped much. But in this game, at this time, my team did not know this. As a result, the outcome of each round was largely random, based on whether we hit the right or wrong bombsite. Needless to say they never succeeded on a single retake. This is the key to why something like this is considered a "noob strat"- The outcome of each round is largely randomized. Neither us or the other team can do much through out the course of the game to help us or themselves. Even more importantly, the other team had gotten to this rank with significantly less work and less effort than we had. Essentially, they were duplicating our success with a fraction of the effort, and offering us no option to fix this within the game. Now, was this a sustainable way to rank up for the other team? Of course not, and I imagine that they are still stuck at MG2 while most of my team has now ranked up past that. But that's not the point. Right then and there, we had no other options, and they almost won against us by pure chance.
While this is touched on in the accepted answer, the aspect of randomness in something like this is not addressed. Noob strats and guns are nooby because they succeed from RNG, and not real skill. This makes other players (somewhat understandably) mad, and certainly doesn't help prevent a toxic community.
Edit: I see that this answer warrants some clarification. I didn't want to repeat what was said in the accepted answer, but I'm going to go over some of the same things, and append to them. Specifically, the idea of skill ceilings. Every weapon and strat in the game has a limit to how successful a team or player can be with it. For some strats, see above, the skill ceiling is fairly low. For other strats it is rather high. This is covered thoroughly in the first answer, so I won't go too far into depth here. The point I'm trying to make though is concerning what happens above the skill ceiling. Specifically, a lot of what happens above that is controlled by RNG. Take for example, a spin bot hacker. Effectively, all this hacker has done is taken the RNG and forced it into their favor. A spin botter can easily beat all professional players, even with a poor weapon. This is the key point. Every bad weapon can become significantly better when the RNG works out in favor of that player. Now, at professional levels this doesn't work all too well, because the chance that a P90 spray and pray will kill an AK player faster than the AK player will kill the P90 sprayer is nearly zero. On the other hand, at lower levels, this becomes quite possible. Now, let's look at this from the low level AK player's perspective. Someone who has put almost no work into their game can win against someone who is practicing, trying to get better, and has a chance of becoming a good player at some point in the future. At this point, it's no surprise that the AK player might get mad and start yelling "N00B" in chat.
Two more things:
You asked what guns are considered "nooby guns". The short answer is: none of them. The long answer is that it depends. All guns in CSGO have a legitimate, if very specific, use case. However, in 9/10 cases most of the guns that are called "nooby guns" are not used in this way. A great example of this is the shotguns. Professional players have found success with them in places such as drop room on Cobblestone or ladder room on Train. However, this does not mean that you should buy shotguns and try playing those positions. In CSGO, weapon buys is a strategy that needs to be coordinated by the whole team. If you go ladder room, you need to make sure that your team can and does throw the necessary smokes and nades for you to be successful with a shotgun if no T's come ladder. When your team does this, you succeed consistently because of good coordination and teamplay. When your team doesn't do this, you'll only succeed if you happen to pick those rounds in which terrorists are going to ladder room. You won't be winning from skill, coordination, or game sense, but because of sheer luck. Not only will this make people angry, but it won't help you become a better player or find more success in the future.
Secondly, I'm not attempting to excuse the behavior of the players that react to what I mentioned above with anger or something similar. These people contribute to a toxic community, and in most cases CSGO would probably be a more fun game without them. But they are not yelling because they are bored or enjoy doing so; They are yelling for a somewhat understandable reason.