etymology - Does a gerund always end with "-ing"? If so, why?

After asking what the difference is between a gerund and a participle, I began to wonder if all gerunds end with -ing, since I couldn't think of any that didn't. If they do, why?

Answer

To answer the original question:

Yes, gerunds all end with -ing, simply by definition. A gerund is, in Latin, a form of the verb which can be construed as (i.e. has functional characteristics of) a noun – it can act as subject or object of a verb, for example, or can take a plural ending. In English, the only category which meets this definition are "verbal nouns" or gerunds, which consist of a verb and a special -ing suffix which turns them into nouns. Although they look like present participles, they are morphologically separate, as we will see...

To answer the bounty question:

The gerundial -ing and the present participal -ing are, in fact, two different suffixes.

Let's start with the gerundial -ing. This is related to modern German -ung and modern Dutch -ing. It started life as a suffix forming nouns of action in Old English, usually written "-ung" - "gaderung" (gathering), "ceaping" (buying and selling). These gerunds were originally abstract, but even in Old English they started to develop into nouns of completed action, etc.: "bletsung" (blessing), "weddung" (betrothal). They subsequently developed plurals, and sometimes became concrete: "offrung" (offering). These uses all developed in the Middle English period, and by late Middle English they were well-established, particularly the gerundial use. Essentially, therefore, there never was a competing form for the gerundial -ing. It started off as "-ung" or "-ing" and continued in that form.

The present participial -ing, on the other hand, started off life as the -ende / -ande form that you refer to. It's related to modern German -end, and Swedish -ande. Even in the Old English period, -ende was often weakened to -inde, and by early Middle English there seems to have been a tendency to confuse "-inde" and "-inge" (this is particularly noticable in Anglo-Norman manuscripts of the 1300s.) Northern forms of the language retained -inde forms, however, though the distinction is not particularly obvious in Northern dialects, since both "g-dropping" and the tendency for "d" to be diminished after a preceding nasal consonant in an unstressed syllable (see, for example, the tendency to say "an" or "en" instead of "and" - "rock 'n' roll") means that either -ing or -ind may generally be produced as -in'.

It's possible that, in the later Middle English period, the development and prominence of the gerundial -ing, perceived as a quasi-verbal ending, also helped to strengthen the participial -ing. But their origins are quite separate, with participial -ing as a diminished form of -ande, while gerundial -ing has undergone few changes from its original form.

Information from my own knowledge, and from the OED's entries -ing, suffix¹ and -ing, suffix². Examples taken from OED, because they are excellent illustrations of the points.