Dexos1™ Gen 3 is a global engine oil specification introduced by General Motors (GM). The new specification is designed to meet performance needs of GM vehicles and is an upgrade from the dexos1 Gen 2.
This article is well-written and covers many of the major differences.
I thought this point was interesting:
Are Group II+ base oils allowed in dexos1 Gen 3 formulations?
Group II and II+ base oils are certainly allowed in dexos1 Gen 3 formulations, but GM will allow limited to no read-across for formulations that contain greater than 30 percent group II/II+. This typically applies to 5W formulations and could possibly apply to 0W formulations with a GIII+ or PAO trim fluid.
This leads me to believe that some of the house-brand Dexos approved oils right now are probably using II/II+ to some degree.
I have a feeling that the flagship full synthetic oils from the major brands (M1, PP, Edge, Valvoline) are probably capable of meeting the spec after the SP reformulation, but the generic oils may need to be revisited.