i had a rather chatty mileage plus rep on the phone a few months ago actually read to me the lines in their system about GS, which was:
(nutshelled)

3 ways to qualify:

1. revenue
2. requal
3. nomination from reps

1 is an unknown, but if it's a percentage threshold it makes sense that each year it'll be a bit different. i know that with about $36k in spend (and 165k BIS / 300 EQM) in november (full fare transpac Y/B, one C) I was nominated and rejected. i know two weeks later with $42k in spend I was accepted (without nomination).

2 is published, she read it to me, and has been stated on here a few thousand times. 50k miles in the good letters of the alphabet

3 she didn't have much info from but comes down to communication between booking agents, travel agents, area reps, station managers, etc. in my opinion this could account for why certain people are approved on less spend or smaller markets and accounts for certain CEOs and travel reps getting the nod to help bolster united support. in the same way any employee in the right position gets free baseball tickets, gifts, candygrams, etc.

but in the end, i'm pretty sure there's a computer algorithm out there that automatically converts your account for initial qualification once you reach a certain threshold, and i don't think miles is in that algorithm. beyond that, i think when your account is reviewed by hand (upon nomination/submittal) it's probably a combination of what you've done as a reward and what your habits are as an incentive. ie, if you book two full F RTs transpac by june,they might throw you GS just in hopes you'll book a 3rd or more.