GIMP doesn’t apply a single Levels adjustment to all layers at once in the usual layer stack, so the practical method is to reuse the same settings on each layer. Two easy ways: 1. **Use Levels/Curves presets** - Adjust one layer with **Colors > Levels** (or Curves). - Save or reuse the tool settings from the **Presets** selector. - Select each other layer and apply the same preset. 2. **Repeat the last operation** - After using **Colors > Levels**, select the next layer. - Use **Filters > Repeat ""** or press **Ctrl+F**. - This repeats the same Levels settings on the newly selected layer. So while you still need to select each layer, you do **not** need to rebuild the adjustment each time. A more advanced workaround is combining layers or tiles into one large layer, making the adjustment once, then splitting them again, but that typically requires extra scripting or file-based workflows.