Why is the "battery not chraging" light on?

charge your battery start your car let it run for 30 min and come back and see if it really isn't charging

Well I did all that. It starts fine, just not sure if we need to do something like reboot I don't know..

Sometimes those rebuilt alternators are bad out of the box, do what crumbob suggested and put a voltmeter on the battery while running, is it at 14V? If not then, your going to have to replace the alternator with another one. If you don't have a meter, you can have the auto parts store check it for you.

The diodes may be going bad. I've seen a few test good but after swapping to a new one the light goes out. The indicator is very sensitive on these gm trucks. They throw the light before the problem is to the point damage is done.