[Campion]

If the heathen get saved without the Gospel, why bother with missionary activity?

Two answers:

1. We don't know that any particular individual or group of individuals will get to heaven apart from repentance, Christian faith, and Christian baptism. That's the way that Scripture lays out for us; the normative path to salvation. God makes the call about whether, when, or for whom He makes exceptions to that normative plan; we don't. However, as for the persons we can't call to that normative plan, or who don't or won't hear it (for reasons we may not completely understand, although God certainly does) ... don't despair. Trust in God's mercy, and give them to him.
2. #1 is irrelevant anyway, because the decision has been made above our paygrade: see the Great Commission. We are required to preach the Gospel and live it whether we think it's absolutely necessary in particular cases or not.

[Knitting A Conundrum]

Above our paygrade indeed!

Our job is to act with loving kindness and spread the gospel, by word, act, example.

God will do what he wants to do.