Gantz: the Abridged Series is an amusing bit created by YouTube user hbi2k starting on May 09, 2008. It was temporarily removed by YouTube, but is back now.
The series originally ended with Gantz: The Abridged Series 18 & 19, and then the two part Gantz: The Abridged Finale.
Click here for the first episode.
And the series finally concluded with the filler chronicles spanning 2 episodes. This had nothing to do with the original main series as it was not canon but Hbi2k promoted it earlier on and finally made it.
He has also created a series called GAS 01 Commentary where he explained how he made all the episodes.