Remove negative terminal wire from battery Using a multimeter on DC setting, or a 12v bulb with leads, test between negative battery post and the negative wire you removed, you should see some voltage or light from the bulb, remove each fuse one at a time and check to see if you still have volts or light. When you no longer have volts you have found your bad circuit.
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.
Check all hoses for any leaks first If you determine that you have a head gasket problem, remove the head and take it to a machine shop to have the ports pressure tested and checked to see if it is warped. If a port is cracked it may be able to be magnafluxed and repaired, if the head is warped, they can plane it to make it flat again. Then replace gasket and reassemble.