GI seals better than composite or amalgam since it is a chemical bond as opposed to a mechanical one. You also don't need to etch/prime/bond, making it less technique sensitive and eliminates the risk of over-drying the demineralized dentin. Since the GI is used as a liner, it's not exposed directly to the oral cavity and won't wash out like a GI restoration that is filled all the way to the cavosurface.