I have an animated gif with something in it I want to remove. I thought I could select all layers(100) and do a Edit > Fill but I cant seem to with the multiple layers selected. In this case where the thing I'm trying to remove is in the same spot in all 100 of those pics what's the best and quickest way to do the Fill on all layers at once ?
If you want to fill the area with the same content in that area you can do as Trevor has written. Before you add the new layer use the frame animation timeline fly-our menu and check new layer appear in all frames. That way you do not need to manually change every frame in the timeline to make the layer visible in the frame's composite image.