Nick, usually a very careful storyteller, goes back at this point and fills us in on what happened with George Wilson after his wife's death, because he believes it is important his readers have this information. While he does not tell us that he heard this account from Michaelis, it is clear that he did from how he narrates the unfolding of events at the garage. To us as readers, this is an account that is filtered first through Michaelis and then through Nick before getting to us. Nevertheless, it sounds believable.
In this account, George is distraught after his wife's death as he tries to put two and two together about the affair and her being run over. He recounts a time a few months earlier when Myrtle came home with a bruised face and swollen nose. He realizes now that her lover was brutal to her. We as readers would remember the swollen nose from the time Tom hit Myrtle during the party that Nick attended in her apartment.
Nick includes this account because it builds sympathy for Wilson and helps us understand his anguish and why he might want to kill the man he held responsible for killing his wife.