What is a gospel issue? How do we recognize one when we see it? D. A. Carson addresses the problem in an insightful essay for Themelios. Here’s one side of the story; it’s worth reading the other:

[B]ecause of the complex entanglements of theology, with a little imagination one might argue that almost any topic is a gospel issue. At one level or another, everything in any theology that is worth the name is tied to everything else, so it is possible to tie everything to the gospel. In that sense, well-nigh everything is a gospel issue.