I don't think your wanting to set that boundary is unreasonable. You can make your point politely and firmly.
I like the approach of letting her know she should let you know when she's coming over - it's YOUR house. And, add the example you said - what if YOU (W) were coming out of the bathroom and saw Gardener standing in your kitchen unannounced?
She should understand, and she may wonder why you are wanting to know when she's coming over - does Gardener have someone over there with him of the female persuasion? Hmmmm.
