Yes, GC is for future job.

NIW is a self-petitioned 140, extra flexibility is offered. Even for employer sponsored GC, you do not have to work for the sponsor until your 485 is approved as long as the position is here when GC is approved and you will take the position immidately after your 485 approval. However, in this situation, the case is likely to be transferred to local. And the CIS interviewer will squeenze you because they usually suspect of the existence of fraud.
Right now with AC21, the beneficiary of an employer sponsored 140 are even free to change job after 180 days of 485 submission to a similar position.