CPL is more of a glorified PPL test in a retractable wobbly prop.
Confident nav using headings and times and recording in your Plog.
I don't know much about the General handling side of things in a single because I did mine in a twin.
But from memory its glide approaches, PFL's, Steep turns, and stalling.
The test will more than likely be done in a Piper product if done on a single.
The Cherokee has the aerodynamics of a house brick compared to the Grob. The difference between it and the bigger aircraft that the OP will be moving onto are minimal. Also the style of flying the grob will be different to what they are going to have to use for the CPL.
I have absolutely zero problems with aspiring commercial pilots gliding in fact I look for it on CV's. But from a fixed wing ex instructors point of view my opinion would be that the piper would give better continuation of experience up the aircraft sizes to CPL level than the grob. If after obtain the expensive blue book they are looking for an aircraft to stay current on or fly for fun I would say the grob should definitely be looked at.