Shohin:What I've done in both situations is tell her no and take the object away from both dogs . . . I figure if they can't play nice, they can't have it.
I think this is more likely to make her think she has to growl at YOU. Normally I would not expect a dog who growls at other dogs to automatically transfer that to people, but if you take prized items away from her, then you are very likely to have her guard them from you.
What do you expect her to do instead of growling? Just let the otehr dog take what she has? I am not sure what you are expecting from the dog(s) here (and I'm suposedly an intelligent species so what chance do they have? 
) Is whatshe is doing really that unreasonable? If someone was trying to take something you wanted, would it be unfair of you to say "No, you can't have that"? Or even, "Hey! What do you think you are doing?"
Perhaps Winston is trying to initiate play and Gracie is telling him she is not interested? Try giving them toys simultaneously - if Gracie has a ball, Winston has a ball. If Gracie knows that you will step in on her behalf and remove or distract Winston, she will feel LESS inclined to growl - at either of you.
"Give a man a fire and he's warm for a day, but set fire to him and he's warm for the rest of his life." (Pratchett, Jingo)
"I used to look at [my dog] Smokey and think, 'If you were a little smarter you could tell me what you were thinking,' and he'd look at me like he was saying, 'If you were a little smarter, I wouldn't have to.'" - Fred Jungclaus