Using the polygon tool in Sketch, you can create a very simple gear icon. To do this, go to Insert -> Shape -> Polygon. Click and drag with the shift key down in order to draw a perfect polygon of size 200px by 200px. From the inspector menu, increase the points from 5 to 24. The polygon which now looks like a circle has 24 points. Click anywhere on the canvas to get out of the vector editing mode.
Go to Insert -> Shape -> Oval. With the shift key down, drag to draw a perfect circle of size 160px. Select the circle and position it at the centre of the polygon. Double click on the polygon to enter into vector editing mode. Drag from the top-most point on the polygon downwards so that it intersects the with the circle. Count 3 points in clockwise direction on the polygon and drag the third point down so that it intersects the circle.
Repeat the this process all the way along the circle.
Reduce the size of the circle to about 50px. From the inspector menu, rotate the polygon layer to about 20° clockwise. Select both the circle layer and the polygon layer and group them. This is how a simple gear icon is drawn here inside Sketch.