Really it depends what you are planning with that image and only one layer.
However, You can bump map a layer with itself. You need to add an Aux Input. Click on the question mark, Then the image entry then the layers entry. Once that is established, adjust the settings.
More usually it is used with a greyscale 'bumpmap' where varying shades of grey can give an impression of depth. Not going to be easy for that image. What are you trying to do?
edit: Another couple of screenshots here: https://www.gimp-forum.net/Thread-How-to-Bump-Map
edit again: Op image is https://imgur.com/BayfJvL.png - London - so it is not going to be height contours
However, You can bump map a layer with itself. You need to add an Aux Input. Click on the question mark, Then the image entry then the layers entry. Once that is established, adjust the settings.
More usually it is used with a greyscale 'bumpmap' where varying shades of grey can give an impression of depth. Not going to be easy for that image. What are you trying to do?
edit: Another couple of screenshots here: https://www.gimp-forum.net/Thread-How-to-Bump-Map
edit again: Op image is https://imgur.com/BayfJvL.png - London - so it is not going to be height contours