I had wondered this too when I seen it on some cakes. I know what you mean abuot the gel or icing leeking out the sides but does it not leek through the ribbon and make it look darker at those spots?
I would use: Tylose Glue with a small paintbrush
In a container with lid, put two tablespoons of warm water and add a quarter teaspoon of CMC or Tylose.
2. Mix the powder with the water. At this time the glue appears not to be blending. Don't worry.
3. Put the lid on the container and leave in the refrigerator overnight and the next day you will have a clearly and ready to use edible glue.
For this cake: http://cakecentral.com/gallery/1680548 All I did was use water on the ribbon to get it to stick to the fondant. It might not work the same with butter cream, since fondant becomes a bit tacky when it gets wet.
I then used a dot of icing where the ends overlapped in the back.
If it is fondant then emridsky is right wet ribbon sticks and dries without any marking to fondant.
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