Was there a relationship between the two prior to the murder? Did Getrude help plot the murder? I know there is suggestion in the play that she does not. I would like to discount that. Any thoughts?
I think that Gertrude was complicit in that she saw what was going on and did nothing about it. She saw what happened to her husband, and what it did to Hamlet, and she certainly knew what kind of person Claudius was.
Adding an element like the one you describe is certainly your prerogative, and is a part of what makes Shakespeare's works so relevant even today.
Most interpretations seem to characterize Gertrude as a weak woman who cannot survive without a husband. Her frailty is what allows Claudius to seduce her and acquire the throne when it would seem young Hamlet should have succeeded his father. But your interpretation that she was a party to the murder of King Hamlet would make sense since Shakespeare often wrote women as strong and determined.