I guess that really depends. If by not part of Mileage Plus you mean you can't earn it by flying X miles, you are correct. But, it is taken into account by United in terms of Upgrades, irrops, etc. I would say that it is de facto the top tier of United's "frequent flyer scheme." In fact, my 1K packet came with the GS materials this year, so your statement may actually be incorrect as of today since UA gives out so little info on UGS. Also, you can requalify for UGS by flying 50k full fare BISM.