I haave a 2000 gmc sierra was sitting for a few months and i went to start it but the battery was completely dead

after a long sleep your woke up abruptly ,slow to function right? at least i am,same thing the pcm is a silly man made brain with performance no better than the builder can try to build , make a few starts, drive it f,,r ect itll come to its senseis

if the 2000 sierra is old fashioned enough, there is something called a "Heat riser" connected to the butterfly on the throttle. Might check to see that this is operational, and choking the right amount based on the temperature- (if there still is one)

Charge your battery!

and put some alcohol based water remover in with at least a half tank of gas. Sitting a long time, especially at this time of year, condensation will form and it can add up to a pretty fair amount of water in there

Did you replace the battery? If you are jump starting it leave the cables connected and see if it dies