I believe I do have a timeline of events that you might find revelatory.
Gally warns her brother, Newt, that she has a “super colossal” favour to ask him.
Through cryptic references to Susanna’s pretend husband, Major Noone, Gally asks if Newt would father a child with Susanna, which Gally and Susanna could then raise together.
Newt is reluctant to do so. In his life generally, Newt has no interest in the required activity and never intends to try it. He remarks that there are other ways of achieving the same effect.
At this point, Gally begins to speak more plainly. She had particularly hoped that Newt would do this for them, so that the kid would be the closest possible thing to being Gally’s own child.
This convinces Newt and they agree to the plan. He asks to be part of the child’s life in the role of an uncle.
Gally begins to explain the practicalities of the arrangement by asking if Newt knows Crewe.