Make the gin mixture ahead or let stand at room temp 3 hours.
Cut 4 cucumbers into 1/4-inch-thick slices (about 2 cups). Muddle sliced cucumber and rind in a large measuring cup or bowl. Add gin. Cover and refrigerate 12 hours and up to 3 days.
Strain cucumber mixture into a pitcher; add juice. Slice remaining 2 cucumbers; add to pitcher with sliced lime. Add tonic water just before serving. Serve in ice-filled Collins glasses.