Possibly from Proto-Italic*-ontnos, syncopated from *-ontinos, from Proto-Indo-European*-ont-(“participial suffix”) + *-i-(“suffix of action”) + *-nós(“adjective-forming suffix”). The -undus suffix, first associated with intransitive deponent verbs as sequor (secundus), orior (oriundus), lābor (lābundus), and other relics, then transited from a derivative to an inflectional category, and the gerundive assumed its canonical shape and function.
The sense of necessity (“needing to be read,” etc.) was a pragmatic outgrowth of the more basic meaning: a book that is readable or “disposed” to be read is one that probably should be read. The replacement of -undus by -endus was a late development under the influence of the present participle; older Latin still has vertundus, legundus, etc.
The derivation from gerundive to the later gerund could then be as easy as an abstraction from “ad librum legendum” (“to read the book”) to “ad legendum” (“to read”).[1] Compare -bundus and -cundus.
