Would it be feasible to use graphene in the gearbox of a large turbofan engine?
Well, just because it is stronger than steel does not mean that it is a useful structural material.  For example, the tensile strength of spider silk is higher than steel, but no one is building gearboxes out of spider silk.  For a graphene gearbox, the big challenge as I see it: graphene by definition is one atom thick. You aren't going to build a gearbox which is one atom thick. You can stack multiple sheets together to get something thicker, but the sheets will just slip against each other, so not useful as a structural material.  i.e. you can build a gearbox out of a big chunk of solid aluminum, but nobody tries to build a gearbox by taking 10,000 sheets of aluminum foil and laying them on top of each other.  What you would need is a graphene composite.  E.g. like we make carbon fiber composites today, which consists of carbon fibers held together by a polymer matrix.  Graphene composites are an active area of research, but I don't think they have reached the point where aerospace companies are considering them.