We understand that your problem is: i changed my head gasket n now my car wont start HELP PLS?

First off we don't know what type of engine you are working on. If it has a timing belt I would look and see if the ign rotor is turning. Remove the cap and see if the rotor is turning when you try and start the car. some cars have a cam sensor and crank sensor. Since you said head gasket and not head gaskets I guess it was a 4 cylinder. Look at the timing belt and check all the wires to see if they are plugged in well. Most gm 5.7 have a ground wire attached to the thermostat housing and if it is broken it will not start.