It's and easy and not easy same time.
Rotation in GDL happens by command ROTX ... ROTY ... ROTZ ... where dots - it's an angle. You can set variable instead of write number in script (don't forget to set the variable in parameters list - it can be any word, for example rotationX, and set type of variable as angle) - then you can control rotation while using object in ArchiCAD, by changing this parameter.
You also can use command ROT rotationX, rotationY, rotationZ where you can rotate all three axis in one command
Then. Usually when you save your morph (or anything) as GDL object, ArchiCAD saves 2D script, that shows this object from top. But it static drawing that is not going to update if you rotate X or Y axis. To fix it you need to open 2D script, delete everything you see there. And write instead PROJECT2 3, 270, 2
That means your plan view symbol will be generated from 3D model.
As the first GDL lesson I think that is enough for you. If you would like to move there further - take a look to GDL reference. It's PDF and accessible from Help menu in ArchiCAD.
Cheers.