I dont use gimp (really must try it one day) but if it's pretty much the same as photoshop which I think it's loosly based on then try around 100 amount 0.8 radius and 0 threshold as a starting point.
A lot depends on what the image your sharpening is though, I use different settings on different subjects, and you don't have to sharpen all the image.
Have a look for a book called "real world sharpening by bruce fraiser" it's an eye opener and well worth a read.