Don't know what actually makes up piping get but I can tell you a couple uses and where to buy it. It's a clear sticky edible gel that can be colored and used if your looking for a water affect on a cake or something that might look like stained glass. In it's clear state it can be used to transfer pictures onto a cake - say you have a coloring book page that your little one wants the same picture/design on a cake - you can put wax paper over the page and put piping gel into a parchment bag and trace the picture using a tip #2 - when it's done - flip the wax paper over and place it on your iced cake and gently rub the outline with a soft paint brush to transfer the gel onto your cake. Lift the paper off and wahlah - you can now do a star fill in or whatever you want to do the design. I've also used it to adhere fondant onto a "crusted" buttercream cake - just put about a 1/2 inch high layer of the gel around the bottom of the cake so the fondant will have something to adhere to. You can also put it into icing to give it a flowing texture - for something like leaves. You can get it at most stores that carry cake decorating supplies - Michaels, Hobby Lobby, etc. Hope this helps. I'm sure there are others here that have more great uses for it too. 
Quote by @%username% on %date%
%body%