One of my students once wrote an analysis of this novel that focused on gender roles and she entitled it Get Me a Beer. I loved that title! This was years ago, so I'm sure she would not mind if you borrowed her idea. If you read the themes section of this story at the link below, it will give you some helpful ideas to get started on a thesis statement. While this novel does focus on gender issues, its theme is that women can fight against stereotypical gender roles if they are strong, have good support and respect themselves. You could write about how the various characters react to gender roles and whether or not they transcend these roles - if so, how, if not, why not. For example, you could use Evelyn, whose character is refined by fire.
Ruth and Mr. Adcock also respond to traditional gender roles in differing ways. A feminist critic would probably discount what happens to Mr. Adcock, perhaps thinking something like, "Well, now you know how it feels," but to be truly objective, he also suffers from gender stereotyping. Although his situation is rare, it is realistic nevertheless.
Your thesis statement could focus on how the author uses characterization to show how gender stereotyping can be overcome. Ask yourself what characters overcome their traditional roles and what enables them to do so. Good luck.