Depends on your budget but i presume you are not willing to spend 4000€ for a genuine tuning tool.
Clones are a risk so have a failover plan (ability to recover from a failed flash with Tech2 or a tool that can recover bricked ECUs)
Galletto, MPPS, Alientech, all should be able to write GM ECUs.

You need an editing software, definition files (damos/A2L).... Best start googling stuff.
There is a long road infront of you if you decide to do this yourself.

You did not tell us which engine you have?