GIMP has notoriously bad tools for that (to make a rectangle you have to make a selection with the rectangular selector, rotate it with the rotate tool if you want it rotated, do this and that and then you have to Stroke Selection or Fill Selection With Colour X etc. it's horrible), but Inkscape has a bunch of geometric tools, and you can easily change in what measurements you're working. I just checked this in Inkscape by choosing the Polygon/Star tool, making a star, selecting it with the selector and changing the measurements shown in the selector's toolbar. From there, you can easily modify it. And because Inkscape is a vector editor, it is easily scalable. If you need to use it in GIMP, just export it using the export dialog as a raster image, and then use GIMP to edit it. This is basically how I use Inkscape and GIMP: Inkscape to do stuff, and GIMP to do other stuff.