When the spire closes a person gets the sum of the loot in the spire crystals that both the person and FS have gotten. In other words, if you beat the 1st floor boss, but the FS has only gotten enough points to get the 3rd crystal, you will get the loot only from the 1st 3 crystals. If the FS instead had passed the 6th crystal, you would get the loot from the 1st 4 crystals. I've just written the above to be sure the mechanics are known.
Now on to a gold spire. It's just a special case of the above. If the FS gets 1330 spire points or more, it is considered to be gold that week. Only the people who have reached the top of the spire will get the loot from all 12 crystals. Everyone else will get loot for the crystals they have passed.
At any time you can look to see what each crystal holds, and so determine what you will be getting when the spire closes.