While you’re holding the Shift key, GIMP will automatically draw a straight line between the two points, showing the path that your auto-filled stroke will take. Hold down the Shift key, and move your mouse to the next corner. But don't think that the lack of a price tag means GIMP is lacking. Select your chosen tool, and click at a point where you’d like to have one corner of your shape. GIMP (short for GNU Image Manipulation Program) is a free alternative to Photoshop that more than holds its own. This method has the advantage that it works with any of GIMP’s brush-based tools, from the Paintbrush to the Clone tool, which creates some interesting possibilities depending on the settings you choose. Usually, I wholeheartedly approve of silly things, but you don’t want to damage your tablet’s drawing surface! It’s possible to draw perfectly straight lines in GIMP using one of the many brush tools – and I don’t mean cheating by using a ruler on your drawing tablet, which is silly. If you don’t want to use either of the selection methods, there’s one last option that might work for you. It is a useful tool for Graphic Designers.

It is packaged with most of the Linux distros. As it is open-source, its source code can be downloaded.

It is a cross-platform tool available for different operating systems such as GNU/ Linux, macOS, Windows, etc.

This is probably the most flexible way of creating a rectangle/box/square in GIMP, and it can be combined with Method 1 or Method 3 to create more complex effects. GIMP (GNU Image Manipulation Program) is a free and open-source image editor tool.

Gnu image manipulation