I have an image with multiple layers, and I have to set the same white point on all pf them. how can I do it without selecting each layer at once?
Doing the layers one by one, most tools in Gimp can save their settings. You can save them explicitly by giving them names, but they are also saved automatically (see the
Presets selector at the top of the Curves or Levels tools).
So you would just do one layer and then reuse the settings on the others.
Another method is to load them as tiles and make one single big layer, make the change on that layer, and then split it again to tiles (but I don't know a script that does that directly, there are however scripts that can do this after saving the tiles to individual files).