I understand completely. I’ve implemented carnivore on more than one occasion and if I didn’t have the physique goals I have, I’d likely by eating almost exclusively red meat.
I’d bet you could gain muscle on carnivore by introducing fruit and honey at the right times, training effectively, taking 3-4 rest days per week where your caloric expenditure is very low and getting proper sleep.
It would be a slow process, but a few pounds of lean tissue a year with this approach is very possible.