It's half badge engineering and half splitting hairs on trim and options. Some folks just assume Chevys are longer lasting, others swear GMCs are stronger. It's all the same in the end.
Silverado is essentially cheaper across the trim levels as well. Example: when I order base model work trucks its cheaper to get the Silverado WT as it has less base features and is essentially $1,000 cheaper off the MSRP, usually 2-3k cheaper fleet price when compared to the Sierra PRO model.
When you go higher trim level it's then a weird mix of "depends what you want". If you want a "sporty" looking truck with painted bumpers, 20s, decent cloth interior $66k gets you that in a the RST Silverado vs. $69k on the Elevation trim Sierra, for essenitally same unpinnings but the GMC seats have slightly nicer stitching, more painted interior pieces and all the GMCs have the 6 way tailgates with the step in them thats an option in the Sierras.
When you go fully upstream to the high trims like the Silverado High Country or the GMC Denali/Denali Ultimate it narrows but GMC still starts slightly higher and stays slightly higher. High County you have to option up Super Cruise and 22'' wheels as well as power retract steps which is all a Denali Ultimate standard and cheap to option to a regular Denali. Also the Silverado can't get Mag ride where the Denali does.
99% of Consumers would be perfectly happy with the Silverado at any trim level but a lot of people pay extra to get the GMC on the grill and Denali/Ultimate on the fender badge for stuff they never use. This runs the gamut with the SUVS as well.
Personally I don't like the tech in the GMCs even the new giant screen in the high trim trucks. Always feels clunky and slow to me, plus I find their seats to be uncomfortable as fuck compared to Ram and Ford.