United Global Services ("GS") is the highest tier of MileagePlus elite status. It is by invitation only, reviewed annually -- the invite criteria is not public but seems to require substantial annual spending with UA both in terms of absolute dollar amounts and purchasing higher fare classes (higher cpm - cents per mile). There are also reports of GS status being comped by UA Marketing to VIPs in companies who sign corporate travel deals with UA, and also to those individuals that purchase an annual PassPlus enrollment of $50k or greater. The sole published way to qualify is to become a 4MM, having flown 4 million BIS miles on UA metal (plus additional grandfathered credits at the time of the UA/CO merger).
GS have enhanced services / benefits over 1Ks. There is little said publicly by UA on GS benefits and/or qualification.
Last year's thread, Consolidated Global Services Thread - Qualifications, Benefits and Q & A [2021]


Known facts concerning qualification and eligibility from United:
Quote:
Global Services recognizes the extraordinary dedication of our top flyers. We consider a number of factors beyond the usual Premier-qualifying criteria.
We look at your travel spend on United-operated flights relative to that of other top Premier flyers over recent consecutive years.
Usually, members invited to Global Services tend to consistently purchase premium cabins or fully refundable tickets.

Published Benefits
Full detail of published benefits can be found at https://mystatus.united.com/gs [MP credentials login required, existing GS only]
Shortlist (incremental/different to 1K):

• Dedicated phone number with fastest pickup times, and generally by specially-trained GS agents. Some exceptions occur, especially during busy periods (such as when weather waivers are in effect)
• Dedicated email address for MileagePlus and other inquiries, usually answered personally within hours
• Dedicated check-in at main airport stations, including private Global Reception centers at major hubs, providing priority TSA security access. May bring guests/companions into Global Reception areas or dedicated GS checkin lanes who are traveling on same itinerary as GS member.
• Instrument-supported upgrades (PlusPoints or miles) clear into PN class rather than PZ class for other elite tiers, which usually have much better availability. Instruments drawn from the GS member account and applied to other reservations will also clear into PN class.
• Complimentary Premier Upgrades (CPUs) cleared beginning 120 hours prior to flight departure (versus 96 hours for 1K) for GS member and up to one other traveling on same PNR
• Highest-priority when waitlisted for Upgrades, Award Travel, and Sold-out Flight Availability. GS members clear before any other status level or full-fare customers.
• Priority Boarding, among the first to board the aircraft (currently after unaccompanied minors, people with disabilities and/or need more time boarding, Active-Duty Military, and prior to families with children under age of 2)
• Priority handling in event of IRROPS, including next flight guarantee in case of re-accommodation, overseen by dedicated team of location-based GS Customer Service Agents (identified via gold scarves/ties)
• Mercedes-Benz Tarmac Transfer for tight connections between flights at major hubs (cannot be requested in advance)
• Access to United Arrivals Lounge at SFO when arriving from international flight, regardless of cabin class (no guests permitted) Closed
• Access to Arrivals Lounge at LHR T2 when arriving from inbound UA-operated intercontinental flight, regardless of cabin (no guests permitted, must present Boarding Pass or physical GS card)
• Access to Lufthansa / Swiss Welcome (arrival) Lounges in FRA and ZRH for same-day inbound UA intercontinental flight, regardless of cabin (no guests permitted, must present physical GS card)
• Effective January 2021, GS members who do not have a United Club membership may access United Clubs on domestic itineraries, but no guests are permitted.

Additional Known Award and Upgrade Benefits (many unpublished)

Most benefits noted below require calling a GS agent:

• GS can create saver economy award space (XN) if T inventory is available. United-operated flights only. Update 12 Dec 2018: Only for GS member + 1 companion.
• GS can create saver business award space (IN) if PZ inventory is available. United-operated flights only. Update 12 Dec 2018: Only for GS member + 1 companion.
• GS can upgrade economy awards (XN or YN) into Business / First with PlusPoints if PN inventory is available. United-operated flights only. Can now be completed online/via the app, in most circumstances.
• GS can confirm any waitlisted instrument-supported upgrade (miles/co-pay or PlusPoints) within T-72 hours to flight departure provided at least 2 (two) J class premium seats remain for sale after the upgrade is processed and they are at the top of the waitlist.
• GS are entitled to one Saver Award seat in the same class of service for one segment of the itinerary with their purchase of any premium cabin fare. GS agents can open space if it is not available provided that at least 2 seats remain for sale afterwards. United-operated flights only.
• GS can upgrade a companion with an instrument (PlusPoints or miles) from Economy to Business / First when flying on a paid premium cabin fare, even if PZ or PN is not available, as long as J > 2 after upgrade processed. Companion needs to be ticketed in W class or higher on international cash fare itineraries, or ticketed in X/XN for upgrades from award tickets.
• (Note: Only ONE of the two above options can be exercised per premium fare purchased, and for one segment in the itinerary, per caveats noted below. GS traveler must remain in same itinerary as companion)
• GS can buy up from Basic Economy fares, which is usually forbidden
• GS can access VIP Lounges (former Global First Lounges) in NRT, LHR, and HKG when booked in Polaris Business on an international itinerary. One guest permitted (two guests if a United Club member). Not accessed if booked in United Economy. (Note: VIP Lounges currently closed due to Covid-19)

Reported caveats & YMMV situations on unpublished benefits

• Post 12/12/2018, agents have been instructed to grant only one "exception" when opening award space for a companion. Thus, you can open T->XN for a companion, or you can force a companion upgrade into PN, but you can't convert T->XN and then force clear an upgrade into PN. Some have reported success by requesting this in two separate calls.
• Some agents will allow GS to open economy award space for a companion to allow for an upgrade with an instrument, even if XN or T are unavailable
• According to GS Customer Service, a GS on a paid coach fare using an instrument will clear into Polaris business before a lower-status passenger with a paid PremiumPlus fare (i.e. O, A, R). In other words, for example, a paid O, A, R for a 1K will not give higher priority than a GS with a W fare if both are using 40 PlusPoints. This is true even on the gate waitlist.
• Post 12/12/2019, agents have been instructed that they can only grant a companion exception for one segment per round trip journey. Thus, if you have a roundtrip premium fare, they will only open saver space, or force an upgrade, for one segment. This means you must either waitlist (if upgrading) or use a standard award, for the remaining segments.