I believe I have solved, to an extent, the Mystery of the GT QoS. It is broken apart into multiple sections.
First on the left hand column click "Game Acceleration". Next click the QoS tab. Enable QoS. Choose categorical priority preference: Gaming, Surfing, Streaming, Custom et cetera. Nothing else can be done on that page. I'll come back to and cover "Game Boost" and "Gear Acceleration" later.
This is the thing I was looking for originally, which I thought should've been on the QoS tab but isn't. If all you want is simple general per device prioritization, left hand column click "Traffic Analyzer". Next click the "Bandwidth Monitor" tab. Here is where you find the drag-n-drop per device basic/general priority level. Drag-n-drop the priority level you want onto each device (notice the thin border around the device icon changes color), hit apply. "App Analysis" is optional, on if you want a more granular view, off if not.
Note devices must be currently on for them to appear for priority selection.
Ok back to the "Game Acceleration", "Game Boost", and "Gear Acceleration".
Warning: I have discovered that if you turn QoS on it does NOT automatically turn "Game Boost" on. However if you have both on and you turn "Game Boost" off it will also turn QoS off. If you do this you can simply turn QoS back on and "Game Boost" will remain off.
On the "Game Boost" page is yet another device level priority list.
On the "Gear Acceleration" tab, you can just add devices.
This I think is how it all works. It's a four tier/way prioritization system.
Lowest most basic level, QoS, categorical: Gaming, Streaming, Surfing, Custom et cetera. For many users this level will be good enough. For everyone else, read on.
The next level up is the basic/general drag-n-drop per device prioritization, found in "Traffic Analyzer">"Bandwidth Monitor". More control over who specifically gets how much of the pie.
The next level up, "Game Boost" acts as an override giving gaming packets (specifically) priority to and from a specified device, regardless of that devices priority level as set in "Bandwidth Monitor" and regardless of the categorical preferences set in "QoS".
The highest level up, "Gear Accelerator" acts as an override above the others. Devices activated here will get the absolute highest QoS priority no matter what. Or perhaps it's a QoS bypass?
My thoughts, and a few use case scenarios.
Under QoS you already set categorical priority when you chose the categorical importance: Gaming, Work From Home, Web Surfing, et cetera. If you put Gaming at the top.....gaming packets already get highest priority. In this scenario you don't need "Game Boost" or "Gear Acceleration". However if your household has multiple gamers, you can place the most important users device in "Game Boost", then that persons gaming packets take priority.
Example: QoS, "Gaming". "Bandwidth" Pa's xbox and Jr's ps4 both have highest priority. Pa's xbox is added to "Game Boost". When Pa and Jr are playing at the same time, Pa's xbox packets will get priority over Jr's ps4 packets.
Another idea, say you have "Streaming" (or anything other than Gaming) as the highest categorical priority. You find you have a few minutes to play "that one game". In this scenario if your device is in the "Game Boost" section, "that one game" will temporarily override everyone's "Streaming" priority.
One last scenario. Let's say you have a house full of gamers. Ma and Pa put their devices in "Game Boost", they get gaming packet priority over the kids. One level further....let's say Pa (or Ma) pays the internet bill. That person wants the absolute highest priority no matter what. Their device goes into "Gear Acceleration". I guess.
It's like a QoS mini game of one upping each other in priority.
I read before Merlin explained why (I think it was) fq_codel was removed from adaptive to traditional, was because he could no longer intercept QoS, or something like that? Trendmicro changed the code or something, I can't remember the particulars. Either way the Adaptive QoS is completely closed source. So how in fact, if in fact, all of this actually works, or doesn't.....is a guessing game. Only Trendmicro, and likely to a lesser extent Asus can know, I guess.
Exhale. Now I'm going to shut my brain off, enjoy my new router, and go play "that one game" for a bit.