Well: the Rectangle node makes rectangles. Its parameters are simple enough to understand. If an exact square is required then the wh (width / height) button should be pressed in the area parameter and the width and height values edited so that they are identical.
Though Nuke has a Ramp node, the purpose of which is to create a simple ramp (i.e. gradient), it is redundant as all its functionality is equalled and bettered elsewhere. The Rectangle is the first place to go should a simple ramp be desired. Here the Ramp Type can be changed in the 'Node' tab. Its advantage over the Ramp node is that it offers the option to blend between two colors, rather than one color and black (which is what the Ramp node does).