600 total calories, with 200 the lower limit for both. So, 400 protein, 200 carbs; 300 protein, 300 carbs etc. 600 is just a lower bound and isn’t something you need to stick to religiously.
There are digestible carbs in Greek yoghurt, turnip, rutabaga, parsnip, olives and the suggested brans/flours/grains. I may be forgetting something, but you should be able to work out 200 or more calories from these sources.
I think gelatine would be fine. It doesn’t contain the amino acids that can be harmful in large amounts (homocysteine and methionine) that are in eggs, dairy and muscle meats, and the glycine can be therapeutic. You can make bone broth if you don’t like the idea of powdered gelatine. It’s said to be good for digestion, and is rich in minerals.
The diet takes a bit of getting used to, but it’s worth trying to work out how to properly nourish yourself early on. Many members don’t seem to get on well with ketosis, others suffer from obvious hypothyroid symptoms, and somebody mentioned being diagnosed with a protein deficiency today. Focusing on what you should be eating rather than what you have to avoid might be helpful.