Why is M-Gyrados B-? It is very spammable on offense, second only to M-Metagross. It has a huge attack stat plus mold breaker, allowing it to break sturdy mons, and even taunt M-Sableye and Mamoswine. Speaking of taunt, it has a variety of options. Though it always runs DD plus stab, its last move slot can be earthquake/ice fang/taunt/substitute. EQ and ice fan give it more coverage, great for opposing offense. Taunt and substitute help against stall, the latter also helping Gyrados set up.
After 2 DDs, it out speeds everything bar +2 Kingdra/Excadrill and +1 Pheromosa. This, in combination with it's +2 attack (around 800, depending on investment), allows it to crush all forms of playstyles.
Although it does suffer from 4MSS, you often only need 4 moves depending on the rest of your team. Regardless of what it runs, it is great against the other top offensive megas. It OHKOs Zam, who fails to OHKO Gyrados (unless modest, and/or has traced protean or other offensive ability). It also does not care about Metagross' stab moves, DDing in its face.
Finally, its major typing change gives it unpredictability. Do I use Pheromosa HJK, expecting it to mega evolve? Or HP Electric? The only reliable way is strong electric stab, like Koko or Xurcitre.
EDIT: I'm not sure what the rankings stand for (fit on any team, good with most match ups, niche, etc.), but I believe M-Gyrados is on the same level as M-Alakazam, and I argue that M-Gyrados should move to A-.