Not to get into the rabbit hole that is breadmaking ('cause it's both deeper and weirder than you think), but perhaps I can offer some detail to clarify?
Gluten is stretchy stuff, and its constituent proteins like to form very thin, very strong sheets. Like rubber balloons, and with the force of trapped gases behind them, they can hold a lot of heavy dough. They're great at capturing bubbles and expanding, which is why bread dough looks smooth and just gets bigger, whereas pancake batter lets bubbles drift to the top and pop. Nothing else does everything gluten does. Other protein/starch/gum combinations trap gases with varying effectiveness, often by being too dense and sticky for air to slip through. No problem for quickbreads (pancakes, muffins, banana bread) or some dense cakes, but it's just... not bread.
Some of them also taste terrible. Guar gum's relatively cheap, but can be gross if not processed to remove the beaniness.
When you bake bread, the expanding, heating gases stretch those entrapped bubbles to give lift to your loaf. The hydrated starch, meanwhile, cooks, and will become the structure that supports the loaf once finished. The gluten holds the air to give the first structural scaffold; the baked crust takes over as structure near the end of baking; and those cooked starches, once cooled, do the lion's share of keeping the bread upright and intact while you slice and eat. Ever try slicing into a loaf directly from the oven? Wiggly and unstable. The gluten's still there, still stretchy, and is a key factor in the chewiness of real bread.
You get a similar effect when you make a classic, unleavened sponge cake. Egg whites, whipped, provide the air-trapping protein matrix and structure, a pan lends a hand during baking, and the starch in the cake flour provides the structure once cool. (Without a bread-like crust, it needs to cool upside down, lest it collapse under its own weight. Which is fun to try once.) Look for eggs to do some of the work in some GF recipes.
Eggs - and a lot of GF replacers - work by substituting a foam of bubbles for the gluten sheets. They're inherently weaker, lack the chewiness, and don't self-repair like gluten. Good fix for some situations, not others.
Another method is to use a tangzhong starter, like you'd use for Japanese milk bread. Cook a portion of the flour in liquid first, and it can lend starch structure early in the process. Same idea is why pate a choux (cream puffs, eclairs) works.
As another analogy, you can look at the difference between egg-supported cakes and vegan equivalents. Like with gluten, there's nothing that does everything egg like an egg. Vegan baked goods tend to denser, fudgier, sometimes gummier textures. You can get around some of that with careful gluten development in certain cases, but that's as limiting as using eggs to simulate gluten.
But at some point, as you approach a straight-up egg, the amount of effort necessary to replicate scrambled (or over easy, if you can imagine it) becomes not worth it. It's more optimization within limited parameters (insert your favorite game analogy here) than true reproduction. Same with gluten.
Re: the book I mentioned above, you can see Aki & Alex's flour blend recipes on Serious Eats. In some cases, they're good as a one-to-one replacement, but in other recipes you'll need to modify your techniques. The more bread-like you get, the less you can rely on wheat-based methods. (I don't see their recipe for Focaccia de Recco online, but it's in the book.)