Both Herland and its sequel were originally published in installments in Gilman's monthly magazine, The Forerunner. Gilman published the magazine for seven years, from late 1909 through 1916, writing all of the material herself. She wrote a total of eight novels for its pages. Herland appeared in twelve installments in the 1915 volume, and the sequel followed immediately in the twelve 1916 issues. So the abruptness of the end of the first novel of the pair is explained; the story continued immediately in the next issue, but the title changed.
Gilman did not see the two novels published in her lifetime, but second-wave feminism found Herland sufficiently interesting, and it was published by Pantheon in 1979 (ISBN 0-394-73665-6). With Her in Ourland is considered an inferior work, and was not published in book form until Charlton Press produced an edition in 1997 (ISBN 0-313-27614-5). It is, apparently, an academic pressing, and suffers from Academic Publishing Price Point Disease: an e-book is available for about $15, but the paperback sold new for $27 and the hardcover for $115. (Used copies are much cheaper.) The scholarly apparatus must be really something, to justify such a sticker on an out-of-copyright book.
Greenwood Reprint reprinted the entire run of The Forerunner in 1968, as part of their "Radical periodicals" series, and Google Books has taken page-scans of the University of Michigan's copy of this reprint. These page scans, together with the usual piss-poor OCR job, are available online from the Hathi Trust Digital Library, the institutional face of a consortium of a bunch of universities, libraries, and corporations with digital content to share. You can read The Forerunner from a web browser, or you can download each volume as a PDF.
This leaves me wondering why Project Gutenberg has no digital edition of With Her in Ourland; it would be fairly easy to construct one, starting from the page scans from The Forerunner at the Hathi Trust. One could argue that this would violate Greenwood's presumed copyright on its edition, though I'm not sure how such a "violation" would play out in the courts. Certainly no copyright would be violated by taking a laptop with a scanner into any library that holds originals of The Forerunner, all of which was published Before the Mouse; if there was a copyright issue there, then Greenwood's edition infringes. If I were to type With Her in Ourland into a text file working from the Hathi scans, Greenwood might have a complaint; if I worked from the originals, typing exactly the same text, I would be quite safe. Does anybody in my vast audience know more about these legal issues?