When my truck sits overnight or for an extended period of time (8 hours), when I start it - it labors and chugs and sounds like its going to die. I give it some gas - once driving it seems fine?
Start by having it scanned for DTC codes, even if the check engine light is NOT on, the computer will have stored info.
Do you notice any black smoke out the tailpipe when it is first started? If possible leaking injectors, if not you should run a fuel pressure and leak down test, the fuel system might be low and then draining back all the way too the fuel tank when it is shut down overnight.
Search GMC Sierra 1500 Questions
Are you a UK consumer? CarGurus now has a discussion forum in the UK.
GMC Sierra 1500 Experts