If it's turning over but not starting check to make sure the fuel pump is working, turn ignition key on but don't try to start, you should hear a faint pumping sound or humming for 3-5 seconds as the pump builds up the pressure needed to start the engine. If you hear nothing, check the fuse for the pump.
If fuel pump is working,
Check to see if your spark plugs are emitting a spark. Remove a plug and put plug wire on, have someone turn ignition key and observe (without touching wire or plug) that there is a spark arcing across the contacts.
If you have fuel pressure and a spark the problem could be with the timing belt or chain and gear, if that is the case you should
bring your truck to a certified GM mechanic.