The Problem with Gunbreaker, short and simple.

[Crater]

This is a hidden strength of Gunbreaker, not a weakness or problem. It means that, while No Mercy is on a strict cooldown, you have some latitude to delay Gnashing Fang, Double Down, and Sonic Break by some amount if necessary/forced and they will gradually re-align themselves as the fight goes on.

Obviously that won't come up during an optimized run where your GCDs are known beforehand, usually won't have much effect, if any, in a run where everything goes smoothly and according to plan, et cetera, but part of the reason Gunbreaker tends to have a more smooth play experience than other tanks is that the effect of Skill Speed on its GCD Cooldowns causes its rotation to self-repair a lot of minor mistakes in a way that you don't get on Paladin or Dark Knight.

[Arohk]

I think you're not understanding the issue i'm pointing out, the entire gunbreaker rotation lasts exactly one minute from one No Mercy to the next, but if you have too much skillspeed, you lower that time from 60 seconds to below 59 seconds, then you end up with 3 Erda and have to wait for no mercy to be ready or spend one Erda on Burst strike or waste one Erda by using solid Barrel because you're one global CD too fast.

[Hierro]

I just clip gcds with blasting zone. Keeps me in line enough

[Kromulus]

I think you're not understanding the issue i'm pointing out, the entire gunbreaker rotation lasts exactly one minute from one No Mercy to the next, but if you have too much skillspeed, you lower that time from 60 seconds to below 59 seconds, then you end up with 3 Erda and have to wait for no mercy to be ready or spend one Erda on Burst strike or waste one Erda by using solid Barrel because you're one global CD too fast.

the rotation of GNB does not ever loop in a fight. ever. the rotation every 60 seconds including the exact weaponskills used both in and out of NM are different. while the initial Gnashing, Double Down, Sonic Break, Savage Claw, and Wicked Talon may always be placed the same in the No Mercy burst, the placement of a Burst Strike in NM will change, the combo actions used change, Bloodfest exists which should be used on cooldown, it will never be the same thing per minute.

also, the only affected SKS actions are Gnashing Fang, Double Down, and Sonic Break, not "Bow Shock" as you stated before.
there are no abilities affected by SKS, only weaponskills.

the goal of the rotation is to attain 3/3 cartridges and utilize GF's combo, Double Down, Sonic, Burst Strike to its maximum possible quantity, and then combo from there inside No Mercy. outside of No Mercy, you are building to 3/3 cartridges while keeping Gnashing Fang and Blasting Zone on CD to maximize their usages, and preventing cartridge overcap by using Burst Strike when you are about to overcap. overcapping is a detrimental loss and should be avoided. there is never a scenario where you need to overcap.

not to mention, by going with a GCD tier that is faster than 2.47, you gain a GCD under No Mercy. the damage calculation is determined from the moment an ability is used, and being faster than 2.47 allows you to fit a ninth GCD inside of NM and have NM's damage buff apply to it. going slower than 2.47 will only cause you to guarantee yourself to lose a GCD inside of NM.

there is nothing unclean about the rotation, you just have Gnashing, Double Down, and Sonic reverse drift and come up earlier and you just hold it by one GCD to push it back into No Mercy.
you'll encounter situations where you are 3/3 cartridges with Solid Barrel ready and NM will not be ready until the next GCD, in which is the only time you use Gnashing before No Mercy as that's the only action available to not delay NM and not scuff your burst window. this is what is known as a GFNM window. it happens under 123NM at 3 minutes, 5 minutes, and 10 minutes.

if you're interested in learning more, please seek out Rin's 12 minute GNB rotation and 123NM opener.
https://www.youtube.com/watch?v=Seua-yrZC6s