How hard is it to gain weight eating only animal flesh or is it even possible at all? Is there anyone on the forum who gained a substantial amount of weight on zero carb?
I used to be underweight and gained back a lot of weight as a carnivore. I've found that eventually (if your underweight), your weight stabilizes and you stop gaining body fat for the most part. I've been able to gain more muscle once I started working out. But otherwise it seemed impossible to gain weight. So if you don't plan on doing some resistance training and eating as much as possible (obviously), you will most likely not gain weight if you are not underweight. This seems to be the experience for a lot of other people eating a carnivorous diet, consisting only of meat.
You can certainly build strength and muscle, but I am not sure how many (if any) have gained body fat, assuming they were not underweight and lacked and adequate amount.
Your body will have a stable weight (neither overweight or underweight) as long as you eat enough. If you want to gain a lot of mass, eat a lot and incorporate resistance training.
100 grams of fat has 900 calories, and there are plenty of sources of pure raw fat. Organ and muscle fat and especially bone marrow are the best sources of fat and could easily give you a 1000 or more calories in a single meal, even if that's all you eat. Eggs are a great source of calories, although not everyone is a fan of eggs. Any fatty cuts of muscle meat add more calories than they would if they were just lean, and there are some other parts like brain, tongue, belly etc... which contain higher amounts of fats. In general, focusing on fat for the calories and then the protein for the muscle building is optimal. Obviously macronutrients are not everything, but as long as you are eating organ meats and fat, you should be getting all the vitamins/minerals as well.
Personally, I just started doing resistance training and eating large amounts of bone marrow and enough protein. It worked for me.