Unfortunately, this answer is very generic and asks for an overall change of approach which you (and others) may completely reject. So here come the downvotes. But, without further ado... :)
"Reaping" does refer to salvation (to quote the other answerer, @Kazark). And in fact, if someone does not do good, they will not be saved. This does not mean that salvation is something people merit, as if people could somehow atone for their sins by doing lots of good stuff. This does not make sense in Paul's scheme of things. But it does clearly mean that salvation will not be attained by someone unless they do good, with actual causality not being assumed in this discussion.
I add this answer, which isn't all that different than the one by @Kazark's yet for one particular reason, and that is because the tension exists because in general the argument of epistle is being misread. Paul is not arguing with a people who are saying you have to merit salvation by doing the works of the law; he is arguing with people who say that anyone who wants to follow God rightly needs to obey the law. It's not about righteousness as a bank of good stuff you could (or could not) do, but rather a discussion of how to live the godly, righteous life. It is more a question of boundary markers or definition than it is a question of grace or grace + works. By missing the argument earlier in the epistle, we set up a false tension between obedience and salvation.
This kind of perspective is not unique to me (I certainly did not come up with it). It lies in the general realm of what some people call "The New Perspective on Paul." If you are interested in reading more on this general approach to Paul, I can recommend N.T. Wright's "Paul in Fresh Perspective."