I would gain fat. I don't lose muscle very easily. Don't exactly built it easily either. Endomorphic tendencies, with just a hint of mesomorph.
I'm peaked naturally in terms of muscle growth. If I somehow figured out a way to get to 5% BF naturally, I'd weight about 185lbs. At 5'10". Probably could weigh 190 if I really did everything right, naturally. Maybe 180 if I got desperate and starved off the pounds.
When I take a layoff for health or family or work reasons, I always get a bit fatter. I like to eat. Training helps me stay lean in spite of my gastronomical habits. No training = get fat fast!
I will have to train for the rest of my life, or I'll look like the offspring of Chris Farley and Camryn Manheim.