Alright, I'll bite. *blows dust off DPP knowledge*
Gyarados: Definitely A Rank. It doesn't have a ton of versatility when it comes to the sets it can run. Bulky/Offensive Dragon Dance, Rest + Sleep Talk, Choice Band are the most common and you can usually narrow down what type of set Gyarados might be based on the other members of the opposing team (you can do this for other pokés too, though). But, the Dragon Dance sets that run Taunt are so dangerous for their ability to just flat out sweep the entire metagame at +2 and most of it even at +1. Once Rotom-A and/or their scarfer is gone (sometimes just Rotom-A), Gyarados runs through any stall team.
Dragonite: If this thing isn't S Rank, it is DEFINITELY A Rank. This is just due to sheer unpredictability. There isn't a whole lot in the metagame that can safely come into Dragonite initially. It could be Dragon Dance(Bulky or Offensive), Mixed, Choice Band or even DD + Heal Bell or Agility. The Mixed Dragonite is arguably the most dangerous. The only safe switch is something absurdly bulky on both ends, like Suicune or Specially Defensive Hippowdon. Having reliable recovery in Roost only makes the problem that is Dragonite even bigger; mitigating it's Stealth Rock weakness and becoming a more effective stallbreaker.
Gliscor: A solid B Rank pick, IMO. Offensively, it has trouble sweeping the metagame with it's Swords Dance variants, but where it really shines is in support. The "Britscor" set is a very popular pick on both offense and stall as a solid stallbreaker. Gliscor has the right amount of speed to outrun all the common walls that stall teams employ to prevent things like status, healing, and hazards with Taunt and has the right amount of physical bulk to switch into things like Close Combat. It's typing is amazing, giving it Electric and Ground immunity as well as being immune to (Toxic)Spikes. Roost allows Gliscor to stick around all game, especially with a lack of SR weakness. Earthquake is standard STAB and U-turn allows even stall teams to maintain momentum. Toxic still gets use to cripple the many Water-types that come in to force Gliscor out. Gliscor is also a solid Baton Passer fwiw. And, no matter what set you use, Sand Veil is the most annoying thing ever for anyone facing Gliscor in the sand since, unlike Gen 5, Sand Veil is not banned.
I question Infernape as an A Rank threat a little bit. I just have trouble seeing Infernape as just as much of a threat as Starmie, who is faster, has a better typing, is just as much if not even more of a threat when holding Life Orb, and has a bit more utility. With Tyranitar and Hippowdon being S Rank threats(which I whole-heartedly agree with), sandstorm is a common condition. Infernape with Life Orb, the most dangerous set IMO, is worn down very quickly and Waters like Starmie, Gyarados and Tentacruel are solid answers to it. Even Vaporeon can check it. Infernape requires prediction to do real damage to a team on it's own. It is definitely a solid B Rank pick (and probably at the top of the list as far as B Rank goes). I just have trouble justifying the standard Infernape as A Rank.
Gyarados: Definitely A Rank. It doesn't have a ton of versatility when it comes to the sets it can run. Bulky/Offensive Dragon Dance, Rest + Sleep Talk, Choice Band are the most common and you can usually narrow down what type of set Gyarados might be based on the other members of the opposing team (you can do this for other pokés too, though). But, the Dragon Dance sets that run Taunt are so dangerous for their ability to just flat out sweep the entire metagame at +2 and most of it even at +1. Once Rotom-A and/or their scarfer is gone (sometimes just Rotom-A), Gyarados runs through any stall team.
Dragonite: If this thing isn't S Rank, it is DEFINITELY A Rank. This is just due to sheer unpredictability. There isn't a whole lot in the metagame that can safely come into Dragonite initially. It could be Dragon Dance(Bulky or Offensive), Mixed, Choice Band or even DD + Heal Bell or Agility. The Mixed Dragonite is arguably the most dangerous. The only safe switch is something absurdly bulky on both ends, like Suicune or Specially Defensive Hippowdon. Having reliable recovery in Roost only makes the problem that is Dragonite even bigger; mitigating it's Stealth Rock weakness and becoming a more effective stallbreaker.
Gliscor: A solid B Rank pick, IMO. Offensively, it has trouble sweeping the metagame with it's Swords Dance variants, but where it really shines is in support. The "Britscor" set is a very popular pick on both offense and stall as a solid stallbreaker. Gliscor has the right amount of speed to outrun all the common walls that stall teams employ to prevent things like status, healing, and hazards with Taunt and has the right amount of physical bulk to switch into things like Close Combat. It's typing is amazing, giving it Electric and Ground immunity as well as being immune to (Toxic)Spikes. Roost allows Gliscor to stick around all game, especially with a lack of SR weakness. Earthquake is standard STAB and U-turn allows even stall teams to maintain momentum. Toxic still gets use to cripple the many Water-types that come in to force Gliscor out. Gliscor is also a solid Baton Passer fwiw. And, no matter what set you use, Sand Veil is the most annoying thing ever for anyone facing Gliscor in the sand since, unlike Gen 5, Sand Veil is not banned.
I question Infernape as an A Rank threat a little bit. I just have trouble seeing Infernape as just as much of a threat as Starmie, who is faster, has a better typing, is just as much if not even more of a threat when holding Life Orb, and has a bit more utility. With Tyranitar and Hippowdon being S Rank threats(which I whole-heartedly agree with), sandstorm is a common condition. Infernape with Life Orb, the most dangerous set IMO, is worn down very quickly and Waters like Starmie, Gyarados and Tentacruel are solid answers to it. Even Vaporeon can check it. Infernape requires prediction to do real damage to a team on it's own. It is definitely a solid B Rank pick (and probably at the top of the list as far as B Rank goes). I just have trouble justifying the standard Infernape as A Rank.