Ingenious, I'd like to introduce to another way though, using the GIMP Path tool. It's nowhere near as useful as the Photoshop one, sadly ( you can use it for masks in Photoshop ) but it's designed for making arbitrarily curved shapes.
So, make a new layer or however you work.
Click the Paths button, or tap the B key.
Click to make a rough outline of your shape, don't worry about the connecting lines for now. Using bezier curves ( which is what paths are ) takes some practice - quite often you will find you've used too many points. You must make a point at sharp corners; make one inbetween if there is a very large gap or if there's some complex changes in direction. Use Ctrl-click on the first point to close the shape. Now let's make some curves.
Hold down ctrl, click on a point and drag the mouse away. While you're doing this, hold shift down so that you get the handle on the other side of the point to appear too. You can think of Bezier handles as magnets that attract the line towards them - if you want more detailed information there's tons of it around, bezier curves are popular tools. Anyway, wiggle the points around until you have a nice smooth curve from each corner. Do the same with the other curve points.
Unfortunately unless I've missed something - I am not a habitual GIMP user - you can't actually do anything with a path directly, so now we're going to turn it into a selection. The button for this is in the tool palette on the left.
Once you have a selection...
... then it's easy to see what to do.
Bezier curves take a bit of getting used to - especially visualising how many points you need - but well worth the effort.