You definitely can't provide a reference or recommendation to former GM workers, as per GM internal policy. The only way around this is to get something from GM Legal, you put in the request on your release day I think, and is not guaranteed to be provided.
The training on the matter is very clear, you can't even +1 someone on LinkedIn. I've given recommendations before for people, but it was for the time before I was at GM, and I did not disclose that I was currently working at GM, and made it very clear that my recommendation was based off the time I worked with them not based off my current position.
After everything that just happened, are you really sure you'd like to draw attention to yourself? Seems pretty dumb to me...