how to read gantz manga after anime

If you want to read the Gantz manga this is the chapter that stayed after the anime:

Gantz has 2 seasons. The first season premiered on April 10, 2004 with 21 canon episodes and 5 fillers.

The anime ends in chapter 21 of which is equivalent to the chapters of Manga 89

So if you want to read the manga where the anime continues, you would have to start reading from chapter 90