While I agree that Chomp might feel a little bit outclassed by Zyg just because the latter can just spam 1k Arrows, I would like to nitpick a few things...Alright, I'll throw in my opinion:
Garchomp: A to A-
Don't get me wrong, Garchomp is still a good pokemon, but it hasn't adapted to the meta trends too well lately. Magearna becoming more popular results in it being killed by a fairy move if it can't kill it fast enough with earthquake, Lando-T overshadows him a lot as well, as it is the best ground type and arguably best pokemon in OU, being even more popular than ever. He also really can't damage popular defensive pokemon enough to be effective like AV Tangrowth, Celesteela, and Skarmory most of the time either. Also, the addition of the Tapus did not really help it either, especially with Tapu bulu rising in popularity. You can run poison jab to deal with the Tapus better, but it struggles to do much to them without it, forcing it to run poison jab over a more valuable coverage move, unless he wants to lose to the Tapus. Not only does it have to fight over a slot in a team against Lando-T, it also has the competition of Zygarde, who has Dragon dance and an extremely spammable move in Thousand arrows. It also struggles with common threats like protean greninja with ice beam, and scarf tapu lele. It can still be an great pokemon on a team that can justify it use properly, but it just doesn't seem as good as it once was. Last generation Zygarde was seen as an inferior garchomp, but now, i feel it's the other way around now.
* P-Jab is a really bad coverage move for Chomp... while it does hit tapus and other fairies super effectively (and Tangrowth, sure), just STAB neutral EQ does almost the same damage to both Fini and Lele, only being inferior against Bulu, which should still receive around 60% from LO Fire Fang (and such move will help against Skarm and Steela). And LO Outrage will 2HKO AV Growth most of the time...
* They do not compete as much as u might think for a slot in the team. Actually Zyg and Chomp usually do different things. Chomp is more of a scarfer/offensive fast rocker, while Zyg is usually going with DD or bulky coil sets. They do share a typing and their weaknesses, but besides that I feel they are different.
* Shift Gear Mage has a high chance of being OHKOed by Scarf Chomp, and AV is gonna be OHKOed every single time by a somewhat boosted EQ (LO, Band, even Soft Sand if something like that would be used), so, Magearna rising doesn't affect that much Chomp's viability since that matchup is shaky at best for both of them.
So, I personally think that Chomp should stay A...
See ya