Fathers and sons love each other and work together. Timothy was not really Paul's son, but he worked with Paul as a son works with his father. (See: [[rc://en/ta/man/translate/figs-simile]])
Here "the gospel" stands for the activity of telling people about Jesus. AT: "in telling people about the gospel" (See: [[rc://en/ta/man/translate/figs-metonymy]])