It probably won't make you gain - but I suspect the reason it isn't is allowed is because there'll be more calories in beef than chicken. Also, depending on how it was cooked, there might be more salt, which might cause a water retention effect.
Best to stick to what's in the book. Have you actually got a copy of the pink/yellow books? If not, then you need to ask your CDC for copies. He/she should have given them to you at the start of the diet.