For example :  The grids are not square, or the grids does not show up at all.

If you ever scaled the object in blender’s “Object Mode”, then it will cause such problem. 

It is not Grid Modeler specific. In fact, scaling in Object Mode cause a lot of problems to other addons too.

Since the scale applied in Object mode is not transferred to the object (bmesh data structure of edit mode), the calculation will be wrong.

To solve it, just scale everything in Edit Mode.  (select all vertices or faces, then scale)

I uploaded some slow videos at normal speed in my youtube channel. They are slow and has key pressed shown on screen.

When you are drawing shape, you can rotate the grid plane by pressing P key.