Do you have the old ECM? I am of the opinion that the ECM is coded to the VIN, so you may want to swap it back and see if you are back in business. Also the keys have a chip that needs to be programed / detected. Perhaps this thread would help or provide other clues - https://forum.hptuners.com/showthrea...Program-G6-Key
Sorry, I am sure there could be a multitude of potential issues. However if the lock symbol is showing I would look at areas related to the vehicle security first, as in the key programming. If you know someone with a higher capability scan tool or Tech II you may be able to get a more detailed diagnosis or re-learn. Sorry to say I do not have more ideas on this.