Combine gin, tonic water, rich simple syrup, and rose water in a large heatproof measuring cup; set aside.
Cut gelatine leaves into fine pieces and place in a heatproof bowl.
Add enough gin and tonic mixture to cover and set aside until gelatin softens.
Place softened gelatine mixture over a pan of simmering water and heat until gelatine leaves are fully melted.
Remove gelatine mixture from heat and stir in the remainder of the gin and tonic mixture.
Strain back into the measuring cup.
Pour into a jelly mould or bowl and refrigerate until set.
Un-mould jelly by briefly immersing the mould or bowl in a bowl of hot water, then invert over a serving plate.
For maximum effect, turn off all the lights and switch on a black light, then serve the glowing jelly to thrilled diners.