Gertrude made the decisions that best served HER! She is not Hamlet's victim, rather she is collateral damage to the situation with Claudius and Hamlet. Contrary to what is suggested in post #4, there are no nasty rumors about Claudius killing his brother to gain the throne. The court of Elsinore believes that their king was bit by a serpent while he was resting in garden. No one has any suspicions of foul play. It is only because the ghost of the dead king returns to tell Hamlet the truth that anything else is known. Hamlet proceeds to keep this information to himself and takes on the burden of fulfilling the request of the ghost and avenging his father's murder.
Gertrude is guilty of marrying his brother-in-law, which would have been considered an incestuous marriage in the eyes of the church. She probably married to maintain her position of queen, and she may have been thinking that her marrying Claudius would actually provide stability to Denmark in maintaining her queen-ship within the construct of the new king.
She deserves Hamlet's anger though. She has betrayed what Hamlet sees as an ideal man and marriage -- his mother and his father. What Hamlet finally, directly confronts her with his feelings on the subject, she acknowledges her sins and seems to want to repent. In Act 4 she proves herself loyal to Hamlet in her not revealing Hamlet's act of madness. I don't think she is his victim -- she actually is standing up to Claudius by drinking to Hamlet's success and defying Claudius's command to not drink the cup. Unfortunately, her realization of being posioned comes to late to help Hamlet see what is happeneing in the end.
Yes, I think that Gertrude is part victim. She was a woman, and she likely felt that she did not have many other options. She didn't wake up one morning and say, gee my husband's brother should murder him so we can marry! She felt trapped.
Sorry if this is a double post -- not sure what happened just now.
Anyway, I'd call it another yes-and-no situation. Gertrude gave away too much power, and those to whom she gave it used it against her. She also made at least one bad decision, to throw in her lot with Claudius instead of allying with her son to ensure the health of the state -- which would have included investigating the nasty rumors that Claudius killed his brother.