At the end of the day, the shape of the earth doesn't really matter. You will still have to get up and go to work, or to church, or spend time with family. It doesn't truly impact you, whatever you believe.
As for the gospel, my problem with zealous flat-earthers is they try to evangelize the flat earth over Christ, as if the flat earth will save one from his sins. I say no, preach Christ crucified, and the flat earth should come way after on the list of priorities for one to believe, if one were to come to believe it.
Biblically, the flat earth could be argued to make sense. However, the earth's shape isn't an obvious doctrine one receives from the Bible, and there's a reason for that... because it doesn't matter. God is the maker of heaven and earth, and his Son Jesus is the Redeemer of all mankind; that is what matters.