I turn my motor over for the first time after changing the headgasket...
and all i hear is it struggling...then a really loud pop just like a gunshot....

I have no idea...what that possibly means at all........

I just wanna get ideas on what to do tomorrow cause I can't look at it until later anyway

 

Start lookin for leaks on the ground!, anything that might'a come loose, your spark plugs then double check that timing!.
Those would be the first 4 things I would be looking at.

 

Your timing is off.
Hope you dident bend no valves.