Blend the spices in a spice grinder or mortar and pestle then mix with the salt and sugar. Add the lime or lemon zest if using. Line a glass or ceramic dish that is big enough to hold the salmon and some juice that will drain off during the curing process. Line the dish with a piece of parchment paper with enough to overhang and fold over the salmon. Sprinkle a handful of the cure on the parchment.
Score the skin side of the salmon with a sharp knife, 5 diagonal cuts, running the length of the filet, about 5mm deep, and place the salmon, skin side down, in the parchment-lined dish. If your fillet doesn't have skin, don't worry about it, but don't cut into it, just lay it in the pan. Pour the gin over the fillet and pack the cure around the fillet, making sure to get some along the sides of the fish. Fold the parchment over the fillet, cover the dish with plastic wrap, and refrigerate for 24–48 hours (start with 48 hours your first time and then if you want it less done you can back off on the time the next time you make it). I like to weight the fish with a pound or two of butter or 32-ounce box of broth, just a bit of weight not a smasher.
After curing, rinse the fillet under cold water to remove cure, pat dry with paper towels and allow to air dry under refrigeration before covering with fresh parchment for storage in a plastic bag or storage container. At serving time, slice very thinly, leaving the skin behind.