Cook bones in a large pot for 8 minutes over medium-high heat. Stir in wine, tomato, gin, shallots, juniper berries, peppercorns, and bay leaf and cook 45 minutes over medium heat or until liquid is reduced to 1 1/4 cups. Drain broth through a fine chinois set over a bowl and dispose of the solids. Mix mustard into liquid and reserve.
Cut off any excess fat from the venison and season with salt and pepper. Pour oil in a frying pan and heat over medium-high heat. Cook venison 15 minutes, flipping to brown all sides. Take venison out of pan and reserve in a warm place.
Pour 1/2 cup gin into pan and cook over low heat until warmed through. Ignite gin with a match to burn off excess alcohol. Mix in reserved broth and let boil. Serve sauce over venison.