Problems with gelatine
The problem is that you're using 3 things that are enemies of gelatin:


Acids
Enzymes
Alcohol


These 3 things are very common protein denaturant agents, which means that they break down the protein structure. And well, gelatin is a protein, so adding freshly pressed grapefruit juice (acid + enzymes) and alcohol to gelatin will prevent it from setting. 

You should boil well the mixture of sugar, juice and alcohol before using and rectify volume with water.