using masks in mapping is probably what you would call a staple to creating good high end maps.... so, definitely get in there and play around with masks... they are confusing at first, how they work, why they work when to use them.. when not use them, etc... but it will definitely pay off in the end if you can get a good grasp on using them ...

bump maps are what trick the eye into believing that the 2d image you are looking at is a 3d image .... just so you know
and I'm not a gimp user, I use PS, but I believe what he is referring to is the filter that gimp has.... bump map filter --apologies if I'm wrong and totally speaking on a platform I've no business talking about