Lights will not turn off and battery goes dead over night.
It is possible that you are still in the 3 year 36000 mile warranty . If that is correct then see your dealer or any GMC dealer.
What lights ?
If headlights, I’m thinking maybe a stuck relay - but could be off base.
Of course if it’s under warranty, let a dealership figure it out.
If it’s the interior lights, could be a faulty door switch. Try pressing/jiggling the driver’s door switch, see if that has any effect. It’s usually the driver’s switch that fails first.