All of my gold plated items are finished over sterling silver with a lovely thick plate, no nickel or base metals are ever used in production. I do my own plating in house so have full control over the quality that goes into the finish. Because gold plated items are a touch more delicate than plain silver, do not sleep in your gold plated jewelry or wear next to multiple pieces for extended periods of time. Do not ever use a polishing cloth on plated items as it will damage and remove the finish. To clean, simply use an old soft toothbrush and wash with warm soap and water, nothing abrasive.
Remove light dirt and tarnish: Use an old toothbrush with soap and warm water, scrub thoroughly and towel dry. You can also try a bit of toothpaste as it has a light abrasive that can clean up mild tarnish quite nicely.
Remove heavy tarnish: Line the bottom of a cup with a small sheet of aluminum foil and fill it with steaming hot water. Add salt (sodium chloride) and baking soda (sodium bicarbonate) to the water ( no measuring really, just add a bit of each). Place silver items into the container so that they are touching each other and resting on the foil. You will be able to watch the tarnish disappear. Leave heavily tarnished items in the solution for as long as 5 minutes, Otherwise, remove the silver when it appears clean.
Remove all tarnish: Only use a liquid silver cleaning product if your piece does not have a beautiful contrasting patina in the recesses. It will strip the metal of any color and leave you with a bright clean white finish.
All sterling silver jewelry will eventually tarnish unless its been protected by a lacquer. How much and how fast it tarnishes depends on several factors including your skin chemistry, lotions/perfumes/cosmetics used, and exposure to moisture. Keeping your jewelry dry and stored properly does extend the shelf life of the beautiful finish, but removing tarnish is easy enough if you follow the above instructions.