As a result of graves' disease, i'm gaining weight, would going on a 800 a day calorie diet be safe?

Most people: With graves' disease, unless it is over treated and becomes hypothyroidism, will gain weight -- but some find their appetite stimulated to the point where they gain weight. 800 c/d is more restriction than most people need, unless you are very, very short and totally inactive. Check with a good registered dietitian. It is hard to get adequate nutrition into 800 cal.