Nope! You cannot go from CPT to GC on EMPLOYMENT basis. It is because CPT is F1 which has the intention to return to your home country after completing study. Where as GC has the intention to stay. So clashes of intentions.
However, there is a provision under law where you can change your status to H1B from F1 and then H1B to GC (most of us are already familiar with this process).

PS: Just my opinion. I am not an immigration attorney. Contact immigration attorney to get concrete info.