This truck has a GM transmission so it shouldn’t be THAT expensive.
At rock auto there are no-name kits at $26 and $28, a WIX kit at $36 and the AC Delco kit is about $50. I’ll bet the shop opted for the Delco kit since it’s the OEM, but I would ask them about using a cheaper one (the WIX one would probably be a good compromise). The filter kit should come with the pan gasket.
Also, it looks to me like they’re going to drop the pan and change the filter and do a flush. Just dropping the pan and changing the fluid should be sufficient. The engineers who made your transmission were aware that a drop-and-drain won’t get all the transmission fluid and are fine with that.