She doesn’t, though, does she?
I mean, if the dumb show is a preview of what’s to come in this play, she most definitively does not keep her word. All it takes is a few gifts from the poisoner and her word is toast.
We don’t get far enough into the spoken portion of this play to see this enacted again but it seems pretty clear that the story is the same – so we can be pretty sure that the Queen does not keep her word on this point.
It would seem that Hamlet can only be saying this as a dig on his mother. As in, SHE’LL keep her word as opposed to YOU who married my stupid uncle, who just in case you weren’t in on it, murdered your first husband, you idiot.
Apparently in the first quarto, it’s made explicit that Gertrude doesn’t know and is therefore only complicit in seeking revenge along with her son. This makes Hamlet seem all the more cruel to her. But in the second quarto and folio editions, it is all a great deal more ambiguous. Does she know or doesn’t she? Is Hamlet’s cruelty to his mother justified? Those editions create more questions, which does make this much more artful in a way. It’s interesting to have to decide.