Liberally squirt ice/glass with lemon and lime juice
Add dash of Angostura
Simultaneously pour in 1 1/2 oz chilled gin + enough tonic water to fill glass
I mixed this out of idle curiosity the other day and was suprised how much it tasted like a margarita on the rocks.
BTW I tried this the next day substituting soda for the tonic, and it was so bad I threw it down the drain. Fascinating what a difference a non-alcoholic ingredient in a small amount can make.