Drawing tools in GIMP

FULL VIEW, please.
This is an overview of all the tools you can use to make lines in the GIMP.

The sizes of strokes with the Pencil, Brush and Airbrush were 1, 3 and 9 pixels respectively. You can see they are not very suitable for fine lineart because they leave uneven and splotchy strokes.

The Draw in Ink tool (which I called Pen before - because it looks like a pen.. heh) has different settings, and a lot depends on the pressure on the stylus when you draw on the tablet.
EDIT: In new versions the icon looks like a bottle of ink.

The Paths tool makes very clean and smooth lines. For stroking the paths you can use a regular line or one of the inking tools with their properties (the 2nd example is stroked with the Airbrush tool).

Hope it helps!
If you have any questions or troubles, feel free to ask!
Used GIMP ver. 2.2.8