GUAVA GEL: Prepare the guava gel while the oven is preheating. In a medium sauce pan, bring guava juice and sugar to a boil. make a thick liquid out of the cornstarch and a small amount of water, approximately 3 tablespoons or more if needed. remove guava juice mixture from heat and whisk in the cornstarch paste. Return to heat and bring back to a boil. Boil for one minute stirring frequently. Remove from heat, add butter and stir to combine. Cool on the counter while cake is baking, then in the fridge. Once cooled, stir in red food coloring until gel is the color you desire.