Which episodes of Konjiki no Gash Bell are essential?
According to this page, there are 121 canon episodes.

Filler episodes for Zatch Bell/Gash bell:

31-35
89-100
139-150

Once you reach the end of episode 138, read volume 22, chapter 212 to continue the story properly.
139-150 have parts in them that are canon, but they are not completely canon, they are mostly filler and the manga is better.