The Wilton piping gel is a clear gel to create beautiful effects in cake decoration. Ideal for making drops of water or misty effects, but you could also use it to attach wafer paper pictures to your cake. It sticks well and since it is clear it doesn't stain. Also use the piping gel to cover the top of your cupcake with a thin layer before you place the fondant or marzipan layer on top.
You could also use it as an outline for pictures on your cake. First pipe the outlines with piping gel, then colour in the figure. The outlines will disappear because the gel is colourless. You can use a piping bag to pipe the gel.