Security alarm goes off. Reset, then goes off 40 minutes later. Only twice a day

Hello, is your GMC equipped with only a factory installed anti-theft system? If so, checking your door ajar switches / door latches would be a first good step. A diagnostic tool would be quite helpful here too, checking the "body" related components to see what is going on. If you have, instead, an aftermarket installed alarm, then I would strongly advise to remove that before doing any testing. www.repairpal.com/repair-shops