This will all be a bit of a simplification

Calories are a unit of energy. When were talking about food we’re usually talking about kilocalories, in the US we usually just say Calories (with a capital C) some other countries may list them as “kcal.” It’s the same thing, 1000 calories (with a lower case c) is 1 dietary Calorie/kilocalorie. The calorie is defined as the amount of energy it takes to raise 1ml of water 1 degree Celsius. That might feel like a weird way to think about energy from food, but energy is energy, whether you’re using it to think, move, or just keep your body temperature where it’s supposed to be

The fuels your body uses for most of its energy are sugars. Sugars are a type of carbohydrate, which are chemicals that are basically just chains of different amounts of carbon, hydrogen, and oxygen.

We general group these into 3 main types

Simple carbs (mono- and di- saccharides)- basically sugars. Glucose is a sugar and the one your body primarily uses, most other carbs basically get broken down into glucose. Sugars are easy for your body to break down and use almost immediately. Sucrose (table sugar) fructose (from plants) maltose, dextrose, lactose, basically anything you might see on a food label that ends in -ose.

Complex carbs (polysaccharides)- these are things like starches. They’re basically just a bigger molecule that your body needs a little more time to break down into simpler sugars, so you’re not really going to be getting energy from them for a few hours or even the next day. This is why you might hear about athletes “carbo-loading” with something like a big spaghetti dinner the day before a big event, the energy from that dinner will be available to them in time for that event.

Dietary fiber - these are basically just really complex carbs that your body can’t really break down like cellulose (there’s that -ose ending again.) Some other animals can break them down for energy, like cows do some extra fermentation in their guts which is why they have 4-chambered stomachs that lets them break it down, but we can’t. It’s still important to other aspects of digestion for us but we’re not burning it for fuel.

Related to that is the glycemic index, which is basically a measure of how fast your body can process those carbs. Sugars have a high glycemic index and raise your blood sugar rapidly. Complex carbs have a lower glycemic index and raise it more gradually.

It wasn’t part of your question, but I feel like that obviously leads to some discussion about insulin and diabetes. Insulin is a hormone that regulates how fast glucose (which remember is the basic sugar that your body runs on and everything else pretty much gets broken down into) is absorbed from your blood into the rest of your body. People with diabetes don’t produce enough insulin, which means that sugar just kind of builds up in their blood and doesn’t get absorbed by their body, which, putting it mildly, is very bad. Not gonna go into too much detail on that, because it’s outside of the scope of the question, and we’re also dancing around the limits of my own knowledge.

Also a bit off topic but let’s talk about fat. You’ve eaten more calories than you really need to sustain yourself, what does your body do with the excess energy/sugar? Well it will try to convert some of that sugar into fat to store for later use when you’re getting less calories from your diet that you really need. It’s an emergency store of energy. It’s sort of slow and energy intensive for your body to convert fat back into sugars that it can use, so your body really only starts burning those fat reserves when there’s no other source of energy available.

Which brings around to the last thing you asked about - gluten. Gluten is a protein found in some grains, when you knead dough made from flour which contains gluten you basically get all of those gluten proteins to get all tangled up and form a stretchy web which makes the dough nice and stretchy, let’s it trap gas bubbles from yeast fermenting or chemical leaveners like baking soda/baking powder, and gives bread a bit of a chewy texture.

Just like with fat and carbs, your body can break down protein for energy (and it kind of needs other things from proteins, but that’s neither here nor there) and for most people gluten is just fine, not much different than if they were eating meat or beans or other protein-rich foods.

But some people have medical issues that prevent their bodies from processing gluten properly and eating it makes them sick, so they need to avoid it in their diet.

Gluten-free foods aren’t necessarily low-carb (or low-calorie,) something like a potato is basically all carbs, but has no gluten.

But if you’re trying to create a substitute for something that would normally be made with something like wheat flour (which is what most bread and pasta is made from, for example, and has a lot gluten and tons of carbs) one of the obvious ways to do that is by substituting something else for that flour that doesn’t contain gluten, like maybe chickpeas, and most of those substitutes are going to be lower-carb than flour, so most gluten-free substitutes tend to be fairly low carb, which sometimes makes them handy for people who need to reduce how many carbs they’re eating even if they don’t have any real need to avoid gluten.