Starts up fine then dies. Already checked alternator,battery.and gasket

Could be several things. 1st you need to determine what is causing the vehicle to die, is it loss of spark or fuel? Need to install a fuel guage tester and see if you have good fuel psi when the engine starts to die. Also need to check for loss of injector pulse when the vehicle starts to die and lastly check for spark with an in-line spark tester to see if spark is going away when the engine starts to die. Once you determine which one is failing then you have a better direction as to which system is failing. Then a better diagnostic path can be laid out to chase down the problem. The other choice would be to bring it in to a local repair shop and have it diagnosed and repaired, may actually save you alot of time amd money.