Again, let me start by asserting my own complementarianism and also my allegiance to a denomination which, as far as I know, is currently under no suspicion of going soft on this issue.
Nevertheless, I remain very uncomfortable with making this a gospel distinctive. First, while I appreciate - and indeed sympathise with - the slippery slope argument (for example, that egalitarians today are committed to a hermeneutic which in the next generation leaves no basis for resisting the legitimation of homosexuality), I am not sure that that is a primary concern of a parachurch group in the present. The church, not the parachurch, is God's means of preserving the gospel. For the full range of Christian truth to be preserved, one needs not only a commitment to orthodox doctrine but also a biblical structure for its maintenance and preservation. That certainly seems to be Paul's perspective in the Pastorals. If the track record of egalitarians holding to orthodoxy in the second and third generations is poor, one has to say that that of parachurch groups driven by big personalities without transparent accountability structures and rooted in tending-to-minimal common ground statements of faith, rather than full-blown historic confessions, is equally suspect on this score. If one is going to make complementarianism a gospel issue on the grounds that this is necessary for preserving the faith, then one must also make ecclesiology a gospel issue by the same token. And that brings us back to a point I have made repeatedly over the last year: if the purpose of your parachurch is just to provide resources to help churches preach the gospel, that is fine but then just major on the gospel; if your ambitions are greater, then you need to come clean, be a church and be accountable as a church.
Second, if complementarianism is a gospel issue, then I think one is really saying that it is a matter which touches directly on the credibility or coherence of a simple profession of faith by an ordinary believer and one should act consistent with that view. That surely makes it very hard for complementarians serving with egalitarian colleagues at seminaries which claim to be Christian or evangelical to do so with real integrity. After all, the gospel as they understand it is being compromised in the classroom by their colleagues. Yet I wonder if serving on faculty with the late Roger Nicole would have been so very problematic to many members of the Gospel Coalition. It also seems to press towards a position where church membership must be denied to someone who is an egalitarian. If it is a gospel issue, then it seems that denial of it must in some serious sense be a denial of the gospel. Here, of course, ecclesiology can help us out: one can make a helpful (and I believe biblical) distinction between the qualifications necessary for church membership (a basic, credible profession) and officebearing which defuses some of the practical issues involved. Complementarianism becomes a matter of faithful, biblical ecclesiology rather than an immediate gospel matter.
Third, we have to be careful what we decide to make into gospel issues and not simply allow our own immediate context or admittedly important cultural struggles to be decisive. Luther thought affirmation of the presence of the whole Christ, in, with and under the elements of the bread and the wine in communion, was a gospel issue such that those who denied this were 'of a different Spirit.' Zwingli thought infant baptism was a gospel distinctive, such that he collaborated with the council in Zurich in the judicial drowning of Anabaptists. Indeed, Baptists have to face the fact that most of their theological heroes throughout history have baptized babies and, indeed, thought it a gospel distinctive. I am not saying that they should therefore become paedobaptists (though they could do a lot worse); but I am saying it should give us pause for thought before we start declaring what are and are not gospel issues and distinctives. This has been a perennially tough question for Protestants and needs to be parsed with care.