Default meaning, the Gemini coin will be worthless when you try to withdraw. You could try to ride 8% junk bonds for 3-24 months too.
I am unsure how they are doing it but I presume they are using a technique similar to the "Arbitrage in Dual Classes" for stocks. Basically you bet that the price of the class A and the class B of a stock will eventually merge to the same or a very similar price. In this case I think they do this with the BTC spot and future price.
This technique worked well for stocks until 2008 when they all got REKT in the Volkswagen Class A stock short squeeze and lost everything. One guy went from billionaire to broke within one day and jumped in front of a train... I have the sad feeling this is going to happen again.
As a bonus, Celsius also claims to do Rehypothecation, the thing that caused the subprime crisis.
Have fun "unbanking" yourselves.