What is the origin of the suffix -ing used to form gerunds and present participles?
Why is the suffix the same in both cases?
The two -ing's are actually not the same etymologically. One developed from Proto-Germanic *-ungō, which has survived in contemporary German (packaging — Verpackung). The other -ing developed from Old English -ende, from Proto-Germanic *-andz — again, compare contemporary German (singing — singend) — and goes back to the Proto-Indo-European *-nt- (cf. Greek -ον or Latin -ans).