Summarize:

Fill a bowl with 1 cup (240mL) of water. Add the juice from half a lemon and 1 tbsp (8 g) of baking soda. Stir to create a soaking solution. Soak your nails for 15-20 minutes, then rinse with fresh water. Lemon is a natural stain remover and can remove yellow stains. It's often used as a whitener! Baking soda is also a stain remover. Use a high-grit buffer, such as a 220 or greater. Lightly rub the buffer over the surface of the gel until the dingy layer is gone. You should see the original color of the gel return. This will protect the manicure and help prevent it from getting dingy again. Lightly brush a thin layer of top coat over your nails, then let it completely dry. You can use your normal top coat over the gel polish. Continue to add a layer of topcoat every few days to keep your manicure looking fresh!
Soak your nails in lemon juice and baking soda to remove discoloration. Buff the top of your gel manicure to remove the dingy layer. Apply a layer of topcoat over your manicure after you buff it.