2004 gmc 2500 hd durmax turbo diesal 6.6l smokes white smoke bad

I have a friend who has your same problem with his duramax and come to find out he was have bad fuel injection problema causing it to white smoke at stops so you need to check out your fuel injectors

I take it that you have white smoke when the truck is warmed up? It sounds like you have a leaking head gasket.

Leaking injectors would create black smoke from running too rich.

Not leaking injectors just like clogged up injection if the trucking any getting fuel on one injector then it can cause white smoke