I subscribe to the view that true Christian unity, the unity Christ prayed for, according to John 17, is unity-in-truth, in particular the truth of the gospel. But the idea of 'unity in the gospel' is challenging in certain ways. For example, it troubles me personally when 'gospel' is so narrowly defined that 'unity in the gospel' means 'our church and just a few others' can work together because, not to put too fine a point on the matter, the others (including a host of churches everyone else in the church views as 'evangelical') do not share the same gospel as us. But I am equally troubled by that widespread Anglican phenomenon in which 'gospel' as a slogan is scrutinized for actual content and found to include multiple versions of the gospel, not all of which are coherent with one another! In short, one challenge of 'unity in the gospel' is defining what the gospel is (and is not).
I think this is 'the' challenge of unity in the gospel. There are some other challenges but I will leave them for another day.