What "stackable and repeatable" means is that if you buy more than the gold required for the highest tier, you can start the tier over again.
Let's say, for example, the tiers went like this (they don't, but it's only an example):
Tier 1, 50g
Tier 2, 100g
Tier 3, 200g
Tier 4, 400g
Now let's say you purchased 1000 gold in the qualifying time frame. You would qualify for all four tiers twice (400 gold x2 = 800 gold), plus tiers 1-3 (your last 200 gold, bringing you up to 1000 gold) a third time.
The promotions like this take into account ALL the gold you exchange for in the event time period, not just your individual purchases. So if you purchase 400 gold three times, it counts as 1200 gold (400 x 3 = 1200).