You can use vodka or even bourbon in this cocktail with great results. While the gin is herbal, the vodka will be more neutral and the bourbon will bring extra warmth.
This is a very ginger forward punch, be sure to use a good quality ginger beer. The syrup is spicy, so use as much as you like to taste.
This punch can be partially made ahead, just add the ginger beer and soda water right before serving to ensure maximum fizz.