I'm in fact slightly confused... I don't get what you mean as the mode to "Multiply"...
Hmm... you work with layers right? And know how to use them?
If you do, then there should be a drop-down menu on the layer box that says things like "Multiply", "screen", "overlay", etc. Go to the shading layer, then go to the drop-down menu and select "Multiply".
This is assuming GIMP has a similar layout to Photoshop ^^;