Looks like gObject.Create method was never added, so for now you will need to use the Draw command to create it first. The option to create a Rect is -b but looks like that was never documented either!
draw -p 3 5;// set current point in axes coordinate
draw -n myRect -b;// create a new Rect with name specified by -n
gobject aa=myRect;//attach it to a GObject, or use myRect directly
aa.DX=2;
aa.dy=1.5;