There is a smooth curve option when you free hand draw, check that box and increase the weight (I think) of the lines, that will help a bit. Lower opacity, check "incremental" and go back and forth to gradually build lines. Working in a bigger scale helps too, but other than that I have to say GIMP isn't really great for making very smooth outlines (even using the bezier tool tends to be not so rewarding unfortunately, the lines come out a bit cruddy looking and you can usually tell when something has been inked in GIMP), it was actually one of the reasons I started doing entirely lineless art instead xD
Other than that I would recommend downloading the old openCanvas (google will help you), that's how I used to work my way around GIMP's cruddy lines.
I've used GIMP 2.6 before but only for a limited time, and when I drew there with a mouse...well...let's just say it probably looks worse than your lines
I guess using the curves, paths, and lines tool will do the trick. But Paint Tool SAI has a stabilizer tool to allow you to freely create smooth lines. You can find SAI on their official site for I think $70? or 80? Although you can also find it around the web for free, which is illegal.