The car is basically telling that you just throw can't ecm's at it all day and expect it to work without going on the SPS site, buy a 3 day subscription, and then you need a j2534 interface and tech 2 emulator at least.
Cheapest way if want to do it on your own. Buy a MDI china clone. Here is a link,
it will do reflash. But also consider a mongoose one. In the long run, the mongoose will be more attractive as it will interface more coverage. But it has to be j2534 protocol.
Here is very informational video of what you need to do. In this vid, he is using an autel much like mine. But the gm procedure with a china MDI clone is pretty much the same . Very important, HAVE A STABLE POWER SUPPLY!!