Thomas Gunn gets a call from an old friend to come solve a murder down in Mississippi

Maybe something like this: "A revered New York private investigator, with an unblemished record at solving murders, encounters his toughest case yet when summoned to the Mississippi Delta by the actual killer."

The MC's name shouldn't be in the logline unless critical to the plot best to describe his character and flaw then goal and obstacle.

The logline describes a situation not a plot as we don't know what the MC wants to achieve and how he will do so.
Also how are these murders connected to the MC? In what way is this personal for him? Or is this a strait up 'who done it'?

Regardless it would be good to provide personal stakes for the MC as this is not an Agatha Cristie story with a pre established episodic formula. Charming and interesting in their own right a new franchise of a Poirot esk type would likely be very hard to finance this day and age without the built in fan based audience.

Hope this helps.