What are the possibilities to design a model in Sketchup and to get a valuable G-Code for milling the model?
Export your file to something like CURA, then that will create the G-code for you for milling. Although the primary function is 3D printing many also will allow setup for CNC.
I have Cura
What is not clear for me:
Let’s say that I want to cut a plate of 100mm from a piece of wood of 150mm * 150mm
Normally, when designing the model for 3d printing there is no material around the model because 3d printing is addictif.
But if I want to mill the plate from a piece of wood, how do I have to proceed so that Sketchup knows that it is a substractive work and not an addictif one?
you might have a look at Fabber (getfabber.com)
SketchUp does not export G-code. Generally, users will export DXF files to a CAD/CAM software for g-code creation. As mentioned above, if you are exporting for CNC you should check out Fabber!