They don't. A fully gemed character has the exact same GS like one with similiar gear but no gems whatsoever.
GS is more accurate in representing the "real strength" of a player than ilvl.
The average ilvl is calculated by adding all the ilvls of your gear up, and dividing it by the total number of slots.
That actually means that a ring increases your ilvl by the exact same amount like a weapon even though the ring gives much less benefit. That's the reason why we have these overpowered PvP twinks. They fine tune their gear based on the ilvl cap of the BG bracket. Means, they only equip item slots with high benefit like weapon, chest, pants, etc, and they leave item slots like rings or bracer out because they give lower benefit but would increase the average ilvl in the same way.
If gear brackets are based on GS it would make twinking less effective. A Weapon increases your GS significant more than a ring. GS doesn't work perfect but it's far more accurate than ilvl.
It's not a eternal format.
As I said, it is possible to implement dynamic gear brackets. That would mean that the gear brackets are no longer determined by fixed gear values. Instead the breaking points of the gear bracket would vary, they would depend on what gear the players that are currently in the BG queue possess.
Dynamic gear brackets would make it impossible for twinks to fine tune the gear to a certain bracket. When they are close to the breaking point they would frequently end up in the higher bracket.