With G4M and G3M range the Germans would be able to attack any target in the British isles,that would hamper British war production quite a bit and cause chaos amongst civilian in the countryside.
They could do that with their existing bombers, and generally do it with a heavier bomb load. The problem is that the bombers suffer when attacking without fighter escort, and the G4M and G3M were much more vulnerable to fighters than the Luftwaffe's bombers.