With the release of Ultima IX: Ascension on GOG, the Ultima saga is officially available in its complete form, as celebrated by this video from GOG. While the game would be normally available for $5.99, it's currently discounted down to only $4.19, and includes the kind of extras we've grown used to with GOG, like artwork, avatars, wallpapers and even 14 design documents.
Here's a brief overview from the store page:
Prepare to embark on the final adventure in Ultima--the best selling role-playing saga of all time that has nurtured generations of gamers. Answer the call and return to Britannia for your last epic quest. As the heroic Avatar, only you can save Lord British and his people from the evil Guardian who has devastated and corrupted the land. Valiant combat, magical prowess, and knowledge of the eight virtues are your weapons against evil in Ultima IX: Ascension.
The ninth installment of the Ultima series takes a leap into a vivid, detailed 3D environment that adds a whole new level of interaction to the exploration of the realm. Britannia was always one of the most life-like, well-developed, king-sized virtual lands, but this new perspective makes the experience of “living” in it much more immersive. The combination of a story of epic proportions, solid mechanics--true to the classic cRPG spirit, and that achingly familiar Ultima setting, Ultima IX: Ascension a memorable title that all role-players should experience for themselves.
Ascend to the realm of myth and legend in the final single-player Ultima game!
That said, even the generous GOG audience awards the title a meager 2.5/5, so you might want to consider the title's reception and troubled history before grabbing it.