High end designers try their hardest to keep their wares out of bargain shops (i.e. you won't see Hermes at TJ Maxx). I don't see how Gilt convinced the fashion world to allow them to sell the high end stuff at a discount. Companies like Overstock said 'We'll take all the unsold inventory in your warehouse and sell it dirt cheap since it's costing you money' but the items that Gilt is selling are much higher quality and they already faced a lot of competition from BlueFly.
As a newbie startup, what tactics did Gilt use to convince the fashion industry that they would be successful without cheapening their brands?
I don't know for sure.
Part of my personal speculation is that it had to do with the original founders of Gilt having close ties to the designer labels in NYC, coupled with their ability to pay on time (another rare commodity in the fashion business).
Most labels don't want clear out excess inventory via TJ Maxx, but they also don't want to get stuck with it. Gilt started out as an "exclusive" site, where you had to know someone or get a referral. The concept was framed as getting rare access to limited inventory at a "never again" kind of price. I think this helped position the site as a high-end concept and not a bargain discounter.
Summary: Connections and marketing/spin. A common theme in success stories :)