It’s very hard to just cut out gluten since almost no one eats straight gluten as a major part of their diet, unless they really really love tempeh. My personal hypothesis is that most non-celiac people who do this and get a major benefit actually get the benefit from removing wheat from their diet.

This is consistent with people who entirely incorrectly claim that sourdough bread is GF. Traditional sourdoughs give bacteria and yeasts a lot of time to eat various sugars and starches.

Tempeh isn't gluten, you might be thinking of seitan.

Indeed. Whoops!

Tempeh is fermented soybeans; seitan is wheat gluten.

non-celiacs who this inadvertently benefit from cutting down on easy calories. no matter the theory, it's still just about energy in vs energy out

Huh? This discussion is about GI issues, not weight gain/loss. I don't think I've ever heard anyone say that GI issues are about energy in vs energy out.

There is a moderately well supported theory that most "gluten intolerance" issues are actually intolerance of FODMAPs: certain fermentable sugars and short-chain starches found in a wide range of foods, most notably wheat. If you remove gluten from diet, you have likely also significantly reduced your FODMAP consumption. But gluten itself isn't a FODMAP, and I once baked an experimantal loaf of low-FODMAP bread using spelt flour and vital wheat gluten. (It wasn't very good.)

Went gluten free at my house for quite a while. Nobody lost weight. Ice-cream, chocolate, potato chips, GF pizza crust, all kind of GF junk food out there if you look for it.

Yes indeed. A lot of commercial GF food is, if anything, higher in fat and sugar than the "regular" gluten-containing equivalent. Just because a product is "free from" something-or-other (whether gluten, lactose, nuts, whatever) doesn't mean it's healthy.

We've gone GF at our house because my wife has coeliac disease. But we use very few commercially-prepared GF products; we cook at home from basic ingredients, and simply avoid the gluten-containing grains.

If you basically went gluten free by relying on products from the supermarket "free from" section, I can imagine it's easy to gain rather than lose weight.

Not going to say it works for everybody. I was having a lot of GI issues and was diagnosed with IBS. Decided to try GF before committing to taking medicine 4 times a day, indefinitely...and it just happened to work.

The weight loss was an unexpected side effect.

You can drop the "if". Almost everything sold by a supermarket is junk food. Avoiding junk food at the supermarket is almost impossible.