It should be covered since it is clearly GMs error. Only way to find out is to ask the dealer.

 

Usually, any gasket of that sort, is semi attached from the supplier and part of the handle assembly when installed on the truck. I suppose it's possible the gasket fell off or got removed during handling. Also possible the handle was removed for some other repair and then the gasket was misplaced when reinstalled.
Strange place for rust to start since the whole door panel is elpo coated. Wonder if there was something rubbing inside the door panel ?

 

Is that two rust bubbles or water droplets in the middle of the door skin in the top picture?

 

I'm curious, where is the rust? Is it in the crevice where the gasket should be?

 

The bubbles are rust that is starting to come through. It popped up not too long ago. I live ed in San Diego and rarely got rained on. I was however a mile from the ocean. I contacted the dealer today and they had me send pics for review, since its 200 miles away. Granite Blue, good point on the elpo coating.I don’t even want to pop the panel to look inside and leave evidence of non factory entrance, or marred screws until they check it out. I hope they cover it, because it wont be long before that cancer spreads.

 

That black area just below that rub spot looks like the gasket may be there. If that is a piece of material thats where the gasket is. Then it would be something else.

 

The missing door gasket could have caused that. It may mean replacing the door skin and repaint to take care of it. You may need to be prepared for that. For sure, leave it alone, do not attempt to open the door panel.