paint tool, is selectable at the bottom. You can choose from a circle, square, or diamond. In addition, you can alter the shape of the nib, by clicking on the gray square in the middle of the shape box and dragging it.
The speed slider above that indicates how much the speed of your drag effects the line. At speed 0 the line is one uniform size, at speed 1.0 the line begins with a smaller dot and thickens the slower you drag.
The tilt slider above that, I believe takes magnetic information from a tablet to determine which way your stylus is leaning. I cannot tell you how it works since I don’t use a tablet to ink.
The size slider, again, I believe uses information from a tablet to effect the size of the line based on pressure. The angle slider, again, no clue, sorry. The size slider effects the size of your nib.
So what I will tell you is that the main things I adjust (in order of utility for me) are the nib, the nib shape, the size, and the speed. Another thing I use, which may or not be apparent to GIMP users, is that several of the tools, including the calligraphy tool are able to create straight lines. You merely need to select a point by clicking, which you can undo (Ctrl + Z) if you like. Hold shift and a line appears that you can drag to any point. When you click again GIMP draws a line to the new point with all the qualities of the line you indicated by your nib, nib shape, size, and speed.
Using this method I’ve been able to work-in different line weights, as well as keeping my strokes nice and clean looking. Its not as elegant as laying in strokes with a tablet, but it does give you some options to get similar effects without driving yourself crazy with a paintbrush and excessive strokes/brushes. Good luck and have fun!