Gauge is a thickness scale that runs backwards. Lower number, thicker steel. That is the whole concept, and on a floor register it is the spec that matters most.
Why it matters on the floor and nowhere else
A floor register is not fastened to anything. It sits in the duct opening held by gravity and by the faceplate overlapping the hole. Nothing is holding it flat.
So when someone steps on the middle of it, the only thing resisting that is the stiffness of the steel. Thin stock bows. Once it has bowed a few hundred times it stays bowed, and a register that is no longer flat rocks and rattles every time air moves through it.
The same register on a wall would be fine forever, because nobody is standing on a wall.
The bin-stock problem
The registers sold loose in home centre bins are thin because thin is cheap and nobody checks gauge at the point of sale. It is a part that costs under ten dollars and gets bought in a hurry.
You find out eighteen months later, when one corner has lifted and the damper no longer holds the position you set it to, because the frame it pivots in has flexed out of true.
What we use and why we talk about edges
Our floor registers are heavy-gauge stamped steel with an edge-folded faceplate. That fold is doing as much work as the thickness is.
Folding the edge over turns a flat sheet into something with a lip, and a lip is far harder to bend than a flat edge. It is the same reason a sheet of paper collapses flat and holds its shape once you fold a crease along it.
On the steel access panel we publish the numbers directly: 0.9mm door in a 1.2mm welded frame. The frame is thicker because the frame is what keeps the opening square.
How to judge one in your hand
Almost nobody publishes gauge on a register listing, so here is the practical test. Hold it flat at one end, like a tray.
A thin register will visibly droop under its own weight. A heavy one stays more or less straight. It takes a second and it tells you more than any product description will.
Where thicker stops helping
There is a ceiling on this. Past a certain thickness you are adding cost and weight without adding anything you can feel, and the duct opening limits how much metal can sit in it anyway.
The useful target is simple: stiff enough that it does not flex when you stand on the middle of it. Beyond that you are buying metal for its own sake.