This is a question that I have often been asked when talking about Explorer's gin. I believe there are no hard and fast rules, however your garnish will impact the flavour profile of 'Explorer's Gin.'
Therefore it is worth understanding the profile. Our profile is soft, designed to be the opposite to some of the bolder flavours. Both citrus, pepper and verbena have been used each providing their own unique flavours. Citrus on the nose, pepper to the taste with verbena providing the silky creamy notes, but at its core a London dry gin.
Therefore using a slice of lemon will do both the gin and your palate a disservice.
WHAT WE HAVE USED
The simple option is to peel the rind of either a red grapefruit or orange, ensuring both are not the waxed options (super markets often wax their fruit). Then slice the peel into thin strips, twirl and quickly warm into shape over heat.
The heat will release some of the oils within the rind, enhancing the profile of the gin. Dehydrated grapefruit or orange will also work if you can wait that long!
The second method uses both the peel with Sichuan pepper. To release the pepper flavour is easy, heat them in a frying pan then take of the heat. These feisty peppers with the orange will bring your gin and tonic to new heights.
By knowing the botanicals used in a gin, choose the garnish that you think will complement or enhance those ingredients. Enjoy and have fun.