I'm so sorry to hear about the problem with your little Gwen.
How much exercise do your dogs get? A lot of the time households with Yorkies have them indoors a good deal of the time because they are so small, cuddly and love to hang out in our laps. (I love that part). I think we forget that these little guys need tons of exercise.
Dogs have an instinct to migrate and an instinct to hunt. Both involves lots of intensive exercise. If this instinct is not met, dogs become agitated and frustrated. Frustration causes them to act out.
We have seven dogs (4 Yorkies and 3 Biewers) and all female except one male and all are not spayed or neutered. They all get along well with each other. I do notice a big difference in their demeanor if they don't get a chance to get out and run their little legs off until they are worn out and ready for naps. If they can't get out and run on days I am not able to let them, they seem figity and a little nervous, and days they get plenty of exercise they are mellow and relaxed.
Perhaps you could try increasing exercise, make sure it is really vigirous and see if it makes a difference in Gwen's behaviour towards the others.
Good luck. I know how heartbreaking it would be to rehome a beloved dog.
Carolyn