First, we need to construct protractor shape to measure angles. So, we need two half circles (using circular sector [icon]/images/ggb/toolbar/mode_circlesector3.png[/icon]).

Now we have two semi circles. If the smaller one is pure white, then it would look like the shape looks like protractor. So we can have arrows (line segments) to measure angles. Now, create the two semicircles either using toolbar or command (CircularSector).