We must remember that this book was written by a woman who was unhappy with the societal norms of her time. Many of the salient factors of the Victorian Age present in Mill on the Floss seem to be portrayed with the intent to critique prevailing gender roles. I will briefly discuss two examples and their relationship to gender.
One important aspect of Victorian culture in the novel is the tension around what kind of education the children should receive. Tom, a middle-class male, is expected to attain a classical education, including the study of Latin, philosophy, and mathematics. It is clear that his mind is not suited for this kind of intellectual labor. When his father asks him about a geometry book by Euclid, he responds, "Oh, I don't know; it's definitions, and axioms, and triangles, and things. It's a book I've got to learn in—there's no sense in it.” By contrast, his sister Maggie enjoys learning. She is curious and inquisitive, and she has a positive relationship with books. At one point in the novel, she remarks,
But I can tell you almost everything there is in my books, I've read them so many times, and that will amuse you. And I can tell you something about Geography too,—that's about the world we live in,—very useful and interesting. Did you ever hear about Columbus?
Though she excels in this area, Maggie is not supposed to spend her time learning the things that boys are supposed to learn. She is expected to learn domestic chores and housekeeping. These gender roles and the way they intersect with education are an example of a historical factor from the Victorian era and its direct influence on the novel.
The career choices available to each character are also indicative of the era. After the Tullivers lose all their money, it falls to Tom to earn enough to get by. There are few careers available to middle-class women beyond becoming a teacher or governess. Maggie takes in sewing projects in exchange for small sums, but Tom does not want people to know that his sister is providing services for money. He says, "I don't like my sister to do such things...I'll take care that the debts are paid, without your lowering yourself in that way." A woman working demonstrates a masculine trait, but it also suggests that her family belongs to the lower classes. When Tom emphasizes that he does not want his sister to work, he is saying that he does not want to be seen as working-class.
Hopefully, these examples will get you started.