Huh. GM resurrected the single-wire "W-spring" system used back in the '80s on some of the wrong-wheel-drive cars. And still using the leading/trailing shoe system. Nuts.
What diameter is the drum ID? Bigger than 254mm, I hope!
Lisle sells a special tool (50600) for popping that style of spring free from the shoes. I don't own one, but I've seen pictures.
www.amazon.com/Lisle-50600-Spring-Brake-Tool/dp/B000GHZMTQ/?th=1
Thexton also made one, #832. Long ago discontinued. Maybe still be able to get a used one on eBay or Craigslist or somesuch.
www.amazon.com/Thexton-832-Brake-Spring-Release/dp/B09KMC2NZR
The Amazon reviews mention using the Lisle tool on the GMT800 trucks.