It means Get It Now. You can place it on your auctions and it lets people have the option to get it right now. Instead of waiting until the auction is over.
The reason I ask is that I bought brand new Levis and Riders same size and supposedly same measurement. Took them home and the Riders were almost 2" smaller than the Levis.
I can tell you that they are pretty close to 20. I will try to find something to measure them. Also they are a stretchy material as well, so they have some give in them.