Hi Rach, its not really reccomended to sub a gel or paste for the liquid,that being said, I did it once for DS birthday cake, I had the americolor gel and watered it down to the same liquid amount, if that makes sense.

I think its not reccomended as the liquid makes up part of the total liquid amount in the recipe? ITMS!

Hope it works out! (also the wilton red can taste quite bitter )