In a small food processor, pulse peppercorns, fennel seeds, and caraway seeds until coarsely ground; combine with salt and sugar.
Spread out one large piece of plastic wrap; sprinkle with half the salt mixture.
Place salmon filet on top, skin side down. Cover with remaining salt mixture, dill sprigs, and gin.
Fold plastic around salmon; wrap tightly with more plastic wrap.
Place the fish in a dish and refrigerate for 48–72 hours, flipping the packet every 12 hours.
When fully cured, the gravlax will darken in color and be firm to the touch at the thickest part.
Unwrap salmon, discarding the spices, dill, and brine.
Rinse the filet under cold running water and pat dry with paper towels.
Place gravlax skin side down on a cutting board.
With a long, narrow-bladed knife, slice gravlax against grain, on the diagonal, into thin pieces.
Refrigerate any remaining gravlax, wrapped in plastic wrap, for up to 2 weeks.
