what will be gyarados and rhydon in the coming Gym Attackers Tier List?

No. Either will be at top tier 2.
Both for the same one reason: No access to [suitable] [elite] moves.

Gyarados does know Hydro Pump, but he can't learn Water Gun. Although Bite is indeed one of the best quick moves, without STAB and effectiveness it is only 0.64 as efficient as Water Gun. This makes him less efficient than Vaporeon when dealing with fire type (I've tried it many times in battle).

For Rhydon, things go worse. Neither Rock Smash nor Mud Slap is considered a good quick move (in fact, they are somewhat bad, for mediate DPS, low EPS and long CD). Stone Edge is indeed elite, but what does a Mud Slap / Rock Smash & Stone Edge Rhydon do me any good? No direct matchups! Remember that Rhydon's base ATK is only 222. So that only leaves Mud Slap / Earthquake Rhydon a potential killer. But to counter Jolteon? Who put Jolteon in gyms?

In a word, they both have excellent base stats, but due to the lack of [suitable] [elite] good moves, in my opinion, they shouldn't make it to more than Tier 2.

Rhydon has very good base stats but very weak to water and weak to ice. Plus ground is uneffective against flying (dragonite/gyarados) which leave Rhydon only useful against Snorlax among common defenders. Which is already good since he is the only one to have positive match-up vs snorlax. So tier 1,5 I guess
Gyarados is a good attacker, at worse tier 1.5.

Its hard to say, the defender list kinda caught me off guard seeing dragonite on the same tier as omastar.

I think rhydon and gyarados will both be tier 2.

If they continue with their current format, I'll expect both of them at tier 1.5.

I'd venture Rhydon at Tier 1.5 and Gyarados with DB / DP at Tier 1.5 (but Tier 2 with Bite). Will Arcanine drop to Tier 2???