Your thesis statement in this case will merely be your answer to the question of how Gertrude's actions influence Hamlet. If you are not certain how Gertrude's actions influence Hamlet, start by listing her actions, and Hamlet's responses to them, then look for connections and commonalities between your examples. In other words, gather your evidence, then come up with your conclusion, which becomes your thesis, rather than trying to come up with your thesis first, which is really reversing the entire mental process you need to go through when writing an essay. Just because your thesis is the first thing you state in your essay doesn't mean it is the first thing you actually write on the paper, chronologically speaking.
Sean, EssayForum.com