I don’t think the ricin will be for Gretchen and Elliot, I think it will ultimately be for himself. The only way Walt is going to save his family is to turn himself in. In the end, I suspect he’ll take out all the Nazi’s, Jesse will either help him or Walt will let Jesse live after finding out what they did in tonights episode and it will provide a little redemption for Walt and Jesse can take care of Brock. Finally, Walt will turn himself in and take the ricin after he confesses.
Finally, the show ends a few months after Walt’s passing, Skyler will have written a best selling book about her ordeal as the wife of a drug lord (remember, Skyler wanted to be a writer back in Season 1), therein Walt will have, in a way, provided for his family after his death, which is what he wanted all along. And it won’t be with the blood money.