It was originally called Future Tense, under which name CBS expressed a brief interest in the idea.  Greg Weisman later renamed it to Gargoyles 2158, both to avoid confusion with the episode also called "Future Tense" and to make it sound more dramatic. After a revamp of the Gargoyles timeline (and the addition of characters Guardian and Zafiro to the cast), the setting of the spin-off was altered and the show was renamed one last time to Gargoyles 2198. 
Gargoyles 2198 is set in the Gargoyles Universe in the year 2198. Most of the original cast, including Goliath, Elisa, Xanatos, and Brooklyn are long since dead by now. Gargoyles are recovering from their earlier near-extinction state; the United Nations has adopted a "Gargoyle Minority Protection Act" that grants gargoyles "Protected Minority" status, although humans are more inclined to grudgingly tolerate gargoyles than to accept them outright. There are now twelve gargoyle clans formed, with a thirteenth being founded at Queen Florence Island, made up of gargoyle eggs from every clan (for the purpose of making a step forward in uniting the scattered clans).
On March 21, 2198, the eggs at Queen Florence Island hatch, with the gargoyle leaders of all twelve clans and human heads of state all present. However, the Space-Spawn suddenly arrive and abduct all of them, afterwards proceeding to conquer the Earth by stealing the Master Matrix, which powers the entire planet.
Samson, a young gargoyle descended from Broadway and Angela, decides to lead a resistance movement against the Space-Spawn designed to liberate the Earth. He is joined by Delilah (a descendant of the original Delilah who becomes his second), Zafiro (a descendant of the original Zafiro), Nicholas Natsilane Maza (a descendant of the original Natsilane and the Mazas, who was raised by gargoyles and identifies with them), Nokkar (who feels ashamed at having failed to detect the Space-Spawn’s approach in time and wishes to redeem himself), Owen (currently stuck as Owen since Alexander, now Secretary-General of the United Nations, was among the human world leaders abducted), the LXM robots LXM-994 and LXM-1057 (Lexington-shaped robots created by the Lexington-Xanatos Corporation), Demona (who recognizes the Space-Spawn as a larger threat to gargoyles than humanity ever was), and a Timedancing Brooklyn and Fu-Dog. The group eventually splits up, with Nokkar taking some of the rebels (Demona, Zafiro, Nicholas, and LXM-1057) into outer space to battle the Space-Spawn in their own home, while Samson leads the rest on Earth. All stories (at least through 2198) will take place within the Milky Way Galaxy. 
The New Olympians would also have a role in the series, but would not likely be re-introduced immediately. By 2198, they have been revealed to the outside world (which has had time to catch up to their technology). Despite their isolationist background, they are involved enough in world affairs to have sent their leader to Queen Florence Island (and who was kidnapped along with all the other leaders). 
The antagonists will include, besides the Space-Spawn, the Quarrymen (still around if somewhat out of favor; they believe the gargoyles to be in league with the Space-Spawn), the Illuminati (who have joined forces with the Space-Spawn out of self-interest), and Coyote-X (the current Coyote robot, who has decided to expand his plans to conquering the entire galaxy rather than just Earth). 
Greg Weisman has indicated that the end of the galactic war with the Space-Spawn isn't necessarily the end of the series.