What are you trying to make? .xcf file that GIMP produces is usually huge in size (several Mb), which I doubt would be posted online as is. Usually you find files online (for images) in .jpg or .gif format. Those are 'flattened' images; so GIMP characteristics have been removed/altered from it. No more layers.
You can use any good graphic design image as a starting point and guidance. But you have to modify it; meaning that you have to erase or add something to it in order to personalize it and make it 'yours'. So you basically take the .jpg or .gif image and paste it into a new GIMP file, and that becomes your template, and go from there. Your modifications can be made in layers, so you can further modify, move, arrange, or do whatever you need to do on your design.