GMC barely sells 250K trucks a year. They are aesthetically different enough from the Silverado that they have their own parts, and that's the kind of stuff Ford has had trouble getting a hold of. Powertrain is not the issue.
Go with the less popular option, more likely there will be stock available. Not surprising. You could probably get a hold of a Tundra or Titan pretty fast, too.
But personally, I do not want any other truck.