The gerund and the gerundive look similar and have similar meanings, but they are still distinct as any Latin grammar will tell us. But how did classical Latin come to have these two close but distinct (sets of) forms? Did the gerund split off as a substantivized gerundive, or did the two end up being similar despite very different origins, or something different?
Assuming the two developed from a common origin, when did the distinction emerge? I have no idea how well we might know this kind of thing, so any justified estimate (such as "some time between PIE and Plautus because…") is great. And if there are trustworthy scholarly sources stating that we simply don't know, that's a good answer, too.