This is entirely anecdotal, although I could throw some research data and "expert" assessments at you that I think bear this thinking out:
There are lots of churches that would say they are gospel-centered and "missional." The truth is very few are both.
Many churches that are "doing missional," do not demonstrate that the gospel is of first importance to them. At best, the gospel is latent.
And there are some (many?) churches that claim to be missional that aren't. (That whole "we reach out to the community to invite them to come see what we're doing" thing.)
Many churches that talk up and down about the gospel aren't doing jack, missionally speaking. And I would argue that if all you do is talk about the gospel, you don't really get it anyway. Clanging cymbal churches.
So lots of these churches (or just some?) that claim to love the gospel actually don't. (Or they figure an invitation at the end of their "be a better Christian" message qualifies as gospel-centered.)
And of course there are plenty of churches that aren't attempting either gospel-centeredness or missionalization. These are probably the "grocery stores" Keller talked about.
Just thinking aloud here.