When an anime is progressing faster than the manga which it is sourced from is, filler episodes are created to compensate. These often lack the quality level of the original manga, as they are generally written by someone other than the series creator. In many cases, fillers are noted as being quite inferior to the originals that they stem from.
Due to the Gantz needing five more episodes to fill the second season of the anime and not having any more source material to work with, it was decided to create filler episodes. These filler episodes, however, gave way to the filler ending of the anime, something that both many fans and reviewers alike see as lacking for it's complete disconnect with the main series. As a result, the filler arc of the second season is quite loathed by many.
Two new characters, the "Bum Hunters", are introduced to the series at the end of Episode 1 (Hajime Muroto) and Episode 17 (Ryuji Kajiura). These two deranged men beat homeless people to death for fun, and kidnap and brainwash women to sell as sex slaves.
In Episode 22, Kurono is joined by people who were never in the Gantz room in the manga. They keep Kurono under an armed guard whilst they get changed as their mission is to hunt Kurono. As well as the standard Gantz guns, they have access to normal machine guns and grenades for their hunt. At the end of Episode 26, Kurono, carrying the body of Mika Kanda, is seemingly sent back in time to a few moments before he and Kato get hit by a train in the very first episode. This happens despite the fact that Gantz was angry enough to send a team of hunters after Kurono and that time travel has never been an ability of Gantz until this point.
It is still unclear and unknown as to what happened to Kurono, as the screen quickly fades to white as he faces the train and transitions to Gantz disappearing, leaving the ending wide open for interpretation.