Is it possible to apply things like brightness/contrast or hue saturation or even filters to more than one layer at a time in Gimp?
If you install the GIMP Animation Plug-in (GAP), you will have available a "Filter All Layers" command added to the Filters menu. When you execute the command, you will be presented with a list of all filters available in the Procedural Data Base and be able to choose one of them to be applied to each of the layers in your image...
Layer groups have a limited ability to affect multiple layers. The most useful of these is opacity. Operations such as brightness and contrast are currently layer-only operations in GIMP and will not work on layer groups.
One possible work around is to save settings as a preset when adjusting brightness, and other settings. This will allow you to re-produce the same settings on additional layers (although they must still be applied to each layer one at a time).