There was more than one spin-off?
There were three attempts, by my count.
The first, and most succesful, was Crusade, which began with the telemovie A Call to Arms and then was launched as a series proper that lasted 13 episodes. TNT had decided to cancel the series before it had even aired, and meddled constantly to get out of their contract, but I think it is still worth watching.
The second attempt at a spin-off was Babylon 5: The Legend of the Rangers. A two-hour pilot movie called "To Live and Die in Starlight" was produced by the Sci-Fi Channel when they couldn't assemble the funds to simply pick up and continue Crusade. But it aired against, IIRC, and AFC football championship game and was pummled in the ratings. So all that we have is the pilot on DVD.
The third attempt at a spin-off was an anthology series that were to go straight to DVD, called Babylon 5: The Lost Tales. Only the first DVD, "Voices in the Dark," was produced. After that the writer's strike happened, and then Warner Bros. backed off on their promise to fund further installments with bigger budgets. To that Joe Straczynski decided to end the series, and has said he will only continue the franchise in the form of a big budget film.
In addition to those, there were four television movies produced for TNT, as well as a re-edited version of The Gathering which I assume you've already seen. These movies were In The Beginning, Thirdspace, River of Souls, and the aforementioned A Call to Arms.
Hope I'm not repeating anyone.
